Manuals, claim bundles, a year of statements: Kernstra’s JavaScript PDF viewer opens them in milliseconds, even at a hundred megabytes. It reads only the pages on screen and streams the rest while your users work. Then they edit the text itself, mark it up and sign it, on a phone or a workstation, with twenty-six tools, every one reachable from your own buttons, and an SDK API for the document itself.
A PDF editor used by thousands of people every month, an AI form builder and a home-care platform. Three very different products, and the same Web SDK doing their PDF work every day.
A complete online PDF editor, built on the Web SDK.
Its Edit PDF tool is the Kernstra viewer. People add text, highlight, underline, leave notes, draw, sign, place and replace images and fill in form fields in their own files, with true editing of the text already in the PDF.
Documents viewed, marked up and edited every day.
FormBuilder.ai is the no-code, AI-assisted builder for forms, surveys and quizzes. Its team runs its daily document work on the Kernstra Web SDK: viewing PDFs, annotating them and editing their text in the browser.
The PDF back office of a home-care platform.
FLEXXI Care connects families in Germany with self-employed carers and household helpers, booked through an app. Behind it, the back office processes its PDF files with the Kernstra Web SDK.
Pick a capability and watch it happen. Under each one is where it lives in the SDK, so keep our toolbar or drive it from your own.
Edit text. Select a paragraph and the options bar reads its real font and size out of the file. Type, and the lines rewrap inside the block’s own width; move or resize the block and they rewrap again. One undo backs the whole session out.
In the SDKviewer.tools.editTextAdd text. Click where the line should start and type at final size, in Helvetica, Times or Courier or one of 58 bundled faces covering 26 scripts. The text becomes part of the page, and moves, resizes and turns like any placed object.
In the SDKtools.addTextMark text. Drag over words and exactly those words are highlighted, in the color and opacity you choose. The selection bar adds underline, squiggly underline, strikethrough and a comment. Every mark is an annotation you can list, update or flatten from code.
In the SDKdoc.annotationsNotes and text boxes. A note carries its author, one of four colors and a message, and opens in place on the page; a text box puts a typed comment on the page that wraps and grows as you write. The author comes from your configuration, so reviewers are named the way your product names them.
In the SDKdoc.annotationsComments. Every note, highlight and mark in the document is listed in the Comments panel with who made it and when. Reply in a thread, set a status (Accepted, Rejected, Cancelled, Completed), and search or filter by author, kind and status. Threads are written the way Acrobat writes them, so they read there too.
In the SDKdoc.annotations.comments()Draw. Freehand ink with adjustable width, color and opacity, for markup that has to be quick: a circle around a figure, an arrow to a defect, a signature-style initial.
In the SDKdoc.annotationsShapes and arrows. Rectangles, ellipses, lines, arrows, polygons and polylines, with stroke, fill and opacity, solid, dashed, dotted or cloudy borders and ten line endings. Each one stays selectable and resizable by its handles, and each one is an annotation on the SDK API.
In the SDKdoc.annotationsStamp. The standard set from Approved to Witness, a Received stamp with today’s date, your own artwork, or a stamp built in place from text, an image or part of the page. Placed, moved, resized and turned by 90° like any other object.
In the SDKtools.stampImage. Drop a logo, a photo of a defect or a scanned signature onto the page (PNG, JPEG, WebP, GIF or BMP), then move it and resize it by its handles. It is written into the page itself, not laid over it as a comment.
In the SDKtools.imageLink. Turn a phrase or a region into a link to a web address or to another page of the same document, so a reference to Schedule A actually takes the reader there.
In the SDKdoc.annotationsErase. Rub out an annotation, or content on the page itself, with an adjustable eraser. Undo brings it back, because erasing sits in the same history as everything else.
In the SDKtools.eraseFill forms. Text, checkbox, radio, dropdown, list box and signature fields are filled in place, with Acrobat’s number formats, date pickers and calculated fields, and tick, cross and dot marks for forms that are only a printed page. Change, focus and blur events let your application react as people type.
In the SDKdoc.formsSign. Type a name in a rendered style, draw it with a finger or a trackpad, or upload a photo of a wet signature with the paper lifted off, then place it on the line. The dialog keeps up to three for the session, and your application can keep them for longer.
In the SDKtools.signSearch. Every match in the file is marked, with a running position count and next and previous controls. The same search is one call on the SDK API, returning each hit with its page and its rectangles.
In the SDKdoc.text.search()Outline and bookmarks. The document’s own outline in a panel, one click to any section, plus bookmarks you add, rename and remove, from the panel or from code.
In the SDKdoc.outlineAttachments. Files embedded in the document are listed in a panel and can be downloaded, and you can attach new ones, so a schedule or a certificate travels with the agreement.
In the SDKdoc.attachmentsOrganize pages. Rotate, move, duplicate and delete from the thumbnails, or open Manage pages to insert blank pages, import pages from another PDF, reorder by dragging and extract a selection as a new PDF. The dialog’s changes apply together when you are done.
In the SDKdoc.pagesProtect and export. Require a password to open the file, with AES-256, and restrict printing, copying, editing and comments behind an owner password. Then print or download every page or a range, flattened or with comments and fields left editable.
In the SDKdoc.securityThe demo above is an illustration. The one behind this button is the SDK itself: the same twenty-six tools on a sample agreement, or drop in a document of your own and edit it in the browser. Nothing you open leaves your machine.
Which tools appear is a map you pass in configuration. A tool you switch off is gone from the toolbar and the selection bar, and whatever you keep can also be driven from your own buttons.
Distance, perimeter and area on drawings and plans, calibrated to the scale printed on the sheet, so 42 mm on the page reads as 4.20 m on site.
Create fillable AcroForm fields on any page: text, checkbox, radio, dropdown, list box and signature. Name them, set required and format rules, and save a form that Acrobat and every other PDF reader can fill.
Alongside the tools: form filling that keeps Acrobat's number formats, date pickers and calculated fields, signature fields included; one undo history for annotations, text edits and page changes; a Comments panel with threaded replies and review status; thumbnails, an outline and attachments you can edit; a panel listing the document's digital signatures; print, and download either flattened or with comments and form fields left editable.
A PDF describes a page as a list of text objects, each with a font, a size, a color and a position. Kernstra reads those objects, groups them into the lines and blocks a person would recognize, and writes your change back into the same objects. Watch the paragraph opposite: the term is retyped, then the block is moved and narrowed, and every line rewraps in the same face because the text was never replaced. Then take the block yourself.
5.Term and termination
5.1 This agreement commences on the effective date and continues for thirty-six months, unless terminated earlier in accordance with this clause or extended by a written amendment signed by both parties.
await viewer.activateTool('editText'); const editText = viewer.tools.editText; const blocks = await editText.listBlocks(3); const term = blocks.find((b) => b.text.includes('thirty-six months')); if (term) { await editText.setText( term.id, term.text.replace('thirty-six', 'twenty-four'), ); await editText.commit(); // repaints, one undo step }
Most viewers draw a page once and stretch that picture when you zoom in. Kernstra redraws it at the zoom you are at, and past the size one page picture may reach it draws the part you are looking at again at your screen’s full resolution, so text and vector art stay sharp all the way to the 800% maximum, on a retina screen too. Below, the same signed page zoomed to 800% on two screens, down to a five-point timestamp.
A maintenance manual, a scanned claim bundle, a year of statements: the documents that matter in real products are big. Kernstra does not wait for the whole file. It requests the part it needs, opens on that, and fetches the rest behind you while you read.
Measured with SDK 1.0.0 in Chrome on an Apple silicon laptop with the file served locally, so these figures exclude your users' download time. Two figures do not depend on the network: the viewer opened after about 0.2 MB of a 107 MB file had arrived, and even when the whole 107 MB was forced to download before opening, the first page was on screen 544 ms later.
Kernstra measures its own container, not the browser window, so it does the right thing inside a split pane, a modal or a narrow column. A wide container gets a toolbar; a touch screen at tablet size gets rails, tools on the left and options on the right; a phone-sized container gets a bottom strip with the active tool's options in a sheet. You write none of it, and you can override all of it.
1.Scope of services
Northgate will provide line maintenance services for the aircraft listed in Schedule A at the stations listed in Schedule B, for the duration of the term.
Northgate will perform daily, weekly and transit checks in accordance with the approved maintenance program, and will record each check in the technical log before the aircraft is released to service.
Unscheduled defect rectification will be carried out on request. Where a defect cannot be cleared within the turnaround, Northgate will raise an acceptable deferred defect and notify the customer within twenty minutes.
2.Service levels
Northgate will meet or exceed the technical dispatch reliability and response targets set out below, measured monthly across all covered stations.
Every visible part of the viewer is a configuration option: which tools appear and where, which panels exist, the theme, the accent, the icons and the words, how it behaves on a phone. Try a few below and watch the code follow. And if you would rather draw the whole interface yourself, keep the document and drop all of ours.
import { PDFViewer } from '@kernstra/pdf-web-sdk'; const keep = [ 'selectText', 'pan', 'search', 'highlight', 'textHighlight', 'note', 'draw', 'rectangle', 'stamp', 'erase', ]; const keep = ['selectText', 'pan', 'search', 'sign']; new PDFViewer(container, doc, { theme: { mode: 'light', accentColor: '#2F6AFF' }, tools: Object.fromEntries( TOOL_NAMES.map((t) => [t, keep.includes(t)]), ), toolbar: { print: true, download: true, actions: [{ id: 'approve', label: 'Approve', icon: 'done', variant: 'primary', onClick: approve, }], }, sidebar: { pages: true, outline: true, attachments: true, signatures: true, comments: true, }, });
A bar, a ribbon, a rail or none, at the top or the bottom, with what does not fit in a menu or a scroll. Any of the twenty-six tools, buttons of your own beside them, and a navigation bar with page and zoom controls.
toolbartoolbar.actionsnavigationPages, outline, attachments, signatures and comments, each on or off; the pages open at the start or not; beside the document or over it.
sidebarlayout.panelsLight, dark or the system's, your accent everywhere, 99 design tokens for color, type, spacing and radius, and an icon of your own for any button.
theme--ks-*toolbar.iconsOn a phone the tools sit in a sheet, a dock, a split or a floating button; a touch tablet gets rails. Touch sizing when you want it, and text editing inline or in a sheet.
layout.phonelayout.tabletlayout.pointerWhich ways to sign are offered, who annotations are by and in which colors, which deletions ask first, print and download, and an event for every change.
tools.signannotationsconfirmationsviewer.on()English ships. Replace any string the viewer shows, or add your own languages, and the viewer uses them throughout.
localeSwitch our toolbar off and draw your own. Every tool can be switched on from your buttons, and the SDK API drives the same document.
toolbar: { enabled: false }Give the viewer a container and a document and you have a working editor in a few lines. Or take the same document object and drive it entirely from your own interface: pages, text, annotations, form fields, search, security and saving are all reachable without rendering a single button of ours.
import { Kernstra, PDFDocument, PDFViewer } from '@kernstra/pdf-web-sdk'; await Kernstra.init({ license: import.meta.env.KERNSTRA_LICENSE }); const doc = await PDFDocument.fromUrlProgressive( '/agreements/nga-2026-0417.pdf', ); const viewer = new PDFViewer(container, doc, { toolbar: { layout: 'bar' }, sidebar: { pages: { defaultOpen: true } }, }); // that is the whole integration
import { useEffect, useRef } from 'react'; import { PDFDocument, PDFViewer } from '@kernstra/pdf-web-sdk'; import { initKernstra } from './kernstra'; // calls Kernstra.init once export function AgreementViewer({ url }: { url: string }) { const ref = useRef<HTMLDivElement>(null); useEffect(() => { let doc: PDFDocument | undefined; let viewer: PDFViewer | undefined; let canceled = false; (async () => { await initKernstra(); const opened = await PDFDocument.fromUrl(url); if (canceled) return opened.close(); doc = opened; viewer = new PDFViewer(ref.current!, doc, { mode: 'full' }); })().catch(console.error); return () => { canceled = true; viewer?.destroy(); void doc?.close(); }; }, [url]); return <div ref={ref} style={{ height: '80vh' }} />; }
// no interface of ours on screen at all await doc.pages.insert({ position: { after: 0 } }); await doc.pages.rotate(2, 90); const [hit] = await doc.text.search('termination'); if (hit) { await doc.annotations.create(hit.pageIndex, { type: 'note', rect: { x: 500, y: 575, width: 20, height: 20 }, contents: 'Confirm the response window covers the night stop.', }); } await doc.security.protect({ ownerPassword, // from your server, not your bundle permissions: { modify: false, print: true }, }); const bytes = await doc.save();
new PDFViewer(container, doc, { // switch off what this product does not need tools: { managePages: false, security: false }, toolbar: { layout: 'rail', // bar · ribbon · rail · none print: true, download: false, }, theme: { mode: 'auto', // light · dark · auto accentColor: '#2F6BFF', // your brand, everywhere }, layout: { phone: 'sheet', // sheet · dock · split · float · none pointer: 'coarse', // 44px targets, no hover }, annotations: { author: 'Elena Marsh' }, });
An afternoon of code on a free development key, then one business day for us to approve the production license.
KERNSTRA_LICENSE=<dev key>Sign in with Google or an email link, no password. Your organization starts with a development key: free, never counted, on localhost and any HTTPS host, with a watermark.Open the console (opens in a new tab)Kernstra.init({ license })One package, its TypeScript types included, in any framework or none. Initialize once, before the first document.new PDFViewer(el, doc)Any element with a height. Choose the tools, panels and look in configuration, or drive the document from your own interface.KERNSTRA_LICENSE=<prod key>Request a production license for your app in the console. We reply within one business day; once the order is signed and paid, swap in the key. Same code, no watermark.Request a license (opens in a new tab)Anywhere your users download a PDF, change it in another app and upload it again, Kernstra turns three steps into one screen, inside your product and on whatever device they hold.
BeforePrint the log, write on it, scan it back
Technical logs, maintenance records and flight manuals marked up where the work happens, on a tablet at the aircraft, with no connection to lose and no scanner in the hangar.
BeforeDownload, annotate in another app, re-upload
Coursework annotated and returned inside the learning platform, worksheets filled on a school tablet, and nothing a student wrote ever uploaded to a third party.
BeforePaper on a clipboard, typed up later
Intake and consent forms completed on any device in the waiting room, where the record never travels to a third party because there is nowhere for it to go.
BeforeRegenerate the contract for one word
Fix a clause at the source instead of regenerating the whole contract, collect signatures on the same screen, then lock the executed copy so it cannot be quietly changed.
BeforeEmail the pack, chase the signature
Statements, loan packs and know-your-customer bundles reviewed and completed inside your portal, then flattened on export so what was agreed is what is stored.
BeforeA 300-page bundle that takes a minute to open
Claim bundles running to hundreds of pages, opened immediately and annotated by an adjuster on whatever device is to hand, with every note carrying a name.
BeforePrint, post, wait
Forms that work in every current browser, completed and submitted from the portal, with no upload step to explain in a privacy notice.
BeforePaper docket, photographed, filed later
Proof of delivery, inspection reports and job sheets signed on a phone in a yard, with a photo of the item dropped straight onto the page, then saved back into your system.
There is no Kernstra server between your users and their files. Opening, rendering, editing and saving all happen on the device.
Request a license for each production app, watch active users against your plan, and manage keys, team and billing yourself. The console is where a development key becomes a production one.
Open the console (opens in a new tab)
We believe a startup, or a small SaaS team, with a strong document idea should never be stopped by the license behind it. Too often that license is a long, rigid enterprise agreement, priced in the tens of thousands a year, and signed before a single line of code has shipped. That model suits large organizations. It fails everyone else. Kernstra is priced so the idea gets built first.
Tell us what you are building, who uses it, your staff or the public, and roughly how many people. We reply with a package and its price within one business day.
Short answers here. Longer ones, with the details, on the security and pricing pages, or on a twenty-minute call.
Ask something elseNo. Opening, rendering, editing and saving happen in the browser, and the SDK never sends or stores document bytes, text, field values, file names or passwords. It makes two kinds of request: a license status check on each load, with your key, a random nonce, the time, a pseudonymous identifier of the browser and a few facts about the SDK and how it loaded, so usage can be counted against your plan; and downloads of the engine, text tools and fonts, which send no data, though the request for a substitute font shows which font a document needed (self-host the fonts, or turn their download off, and nothing does). No telemetry.
An open document keeps working with no connection. The first load on a browser fetches the engine once; after that it comes from the browser’s cache. Each load confirms the license in the background, and if that cannot get through, the last confirmed check carries you for 48 hours, after which output is watermarked until a check succeeds. For networks with no outbound internet at all, an offline license never contacts us and you host the engine yourself.
Plain JavaScript and TypeScript, React, Vue, Angular, Svelte, Next.js, Nuxt and SvelteKit, built with Vite, webpack, esbuild, Rollup, Parcel or the Angular CLI. The SDK draws inside the one element you give it. It runs in the last two versions of Chrome, Edge, Firefox and Safari, and Firefox ESR, on desktop and on iOS, iPadOS and Android.
One browser that loads the SDK with your production key during the billing month, counted once however many documents, pages or sessions it opens. Each production app has its own allowance, the console shows the count per app, updated hourly, development keys are never counted, and staging keys are counted apart, never against your allowance.
Yes, that is how it is meant to be used. A production key works only on the domains in its license, and every load confirms it with our licensing service, so a copy used on another site is refused.
Both. Each of the twenty-six tools is switched on or off in configuration, and one you switch off is gone from the toolbar and the selection bar. Or hide our toolbar and build your own: every tool can be switched on from your buttons, and pages, annotations, form fields, text, search, security and saving all have an API. Added text, text boxes, stamps, signatures and images are made in the viewer, not from code.
Yes. Your change is set in the face already embedded in the file, and a character that face lacks comes from a matching bundled face. By default the whole face is embedded when you save, so the document can be edited again later. Arabic, Hebrew and Indic scripts are shaped; a line that mixes right-to-left and left-to-right text is not yet laid out in both directions.
No. Kernstra does not use PDF.js. It is built on the PDF engine Chromium uses, compiled to WebAssembly and run in the browser, for rendering quality and reliability on real-world files.
An internal tool or back office, used by your own staff with a limited number of users, is $149 a month, and that includes every tool, Edit Text and Add Text among them. A public-facing app, or more users or apps, gets a package priced to its use: tell us what you are building and we reply within one business day. The tools you use never raise the price: every one is in the $149 plan. Billing is monthly with no minimum term, or quarterly, or yearly with two months free, and redistribution or OEM use is on enterprise terms. See how that compares with Apryse WebViewer, Nutrient Web SDK, ComPDF and Syncfusion.
Twenty minutes with an engineer who works on the SDK. Send a PDF from your product beforehand and we will edit it live, on the call, and talk through the integration for your stack.
Confirm the 30-minute window covers the Edinburgh night stop.
Schedule B should list the Belfast hours.