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A PDF suite can run without sending document bytes to a cloud service. The practical design is to keep file objects and generated output in the browser, run PDF parsing and rendering in a Web Worker, and use WebAssembly only for measured hot paths such as image decoding. JavaScript remains the coordinator for the interface, file selection, progress, and downloads.
That is a stronger privacy design than an upload-first tool, but “100% client-side” is a claim about the entire application, not just its PDF parser. You must also account for analytics, crash reports, service-worker caches, remote fonts or scripts, and any network request made by the page. The architecture below shows how to build and verify a zero-upload workflow without claiming that every PDF feature is implemented in WebAssembly.
What a zero-cloud PDF workflow actually means
In a local-only workflow, a user selects a file and the browser receives a File object. The application reads that object with browser APIs, sends work to a worker running in the same origin, and creates a new Blob for preview or download. No document bytes are posted to an application server.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsThat boundary is narrower than “the page makes no network requests.” The application still needs to download its HTML, JavaScript, PDF.js distribution, and WebAssembly binaries when it loads. A strict privacy promise therefore has two parts:
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- Document locality: selected PDF bytes and generated results remain in the browser during processing.
- Network hygiene: the application does not transmit document content through telemetry, error reporting, remote viewers, service-worker persistence, or accidental API calls.
The public material available for the project does not provide an independent audit proving that every byte stays local. Treat “zero upload” as an engineering requirement to test and document, not as an assumption inherited from using a worker.
The architecture: PDF.js, a worker, and selective WebAssembly
PDF.js supplies the PDF engine
Mozilla describes PDF.js as a “general-purpose, web standards-based platform for parsing and rendering PDFs.” Its architecture separates a core layer that parses and interprets files from a display layer that exposes rendering and document-information APIs, with a viewer layer responsible for interface behavior. The stable release identified on Mozilla’s getting-started page is v6.3.289 (2026).
Use the core and display APIs as the engine behind your own interface rather than assuming that the stock viewer is a complete editing application. PDF.js is well suited to loading documents, reading metadata, rendering pages, obtaining text content, and displaying annotations. Operations such as merging files, deleting pages, or writing a new PDF may require an additional client-side writer or a dedicated WebAssembly module; they are not automatically provided merely because a page can render.
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PDF parsing and rasterization can occupy the main thread long enough to make scrolling, drag-and-drop, and button clicks feel broken. PDF.js distributes a pdf.worker.mjs file so parsing can run in a dedicated worker. The viewer must be served over HTTP(S): Mozilla notes that the worker is not enabled for file:// URLs. A tiny local server is enough for development; opening the HTML file directly is not.
import * as pdfjsLib from "pdfjs-dist/build/pdf.mjs";
pdfjsLib.GlobalWorkerOptions.workerSrc = "/assets/pdf.worker.mjs";
const loadingTask = pdfjsLib.getDocument({
data: pdfBytes,
useWasm: true,
useWorkerFetch: true
});
const pdf = await loadingTask.promise;In production, make the worker path match the exact file emitted by your bundler. A mismatched path produces a worker-loading error even though the main PDF.js module imports correctly.
WebAssembly accelerates selected hot paths
PDF.js exposes useWasm, wasmUrl, and useWorkerFetch options. In web environments, useWasm defaults to true and attempts WebAssembly to improve image-decoding performance. That wording matters: it supports a hybrid design in which WebAssembly accelerates selected routines while JavaScript coordinates file handling, state, and rendering. It does not establish that every suite feature runs in WebAssembly.
Keep the WebAssembly binary on the same origin when possible, serve it with a WebAssembly-compatible MIME type, and verify that your deployment does not rewrite or block the binary. If the module cannot load, PDF.js can fall back to JavaScript for supported paths, but startup and image-heavy pages may become slower.
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Build the suite in a controlled sequence
1. Serve the application over HTTP
Start with a static development server and a predictable asset layout:
/index.html
/assets/app.js
/assets/pdf.worker.mjs
/assets/pdf.wasmUse the same origin for the page, worker, and WASM asset during development. This avoids cross-origin worker restrictions and makes network inspection easier. Add a production content-security policy that allows only the script, worker, and asset origins you intend to use.
2. Accept files without uploading them
The input control should hand a File to your application. Validate type and size before reading it, then convert it to an ArrayBuffer. Keep the original File reference only as long as the current operation needs it.
fileInput.addEventListener("change", async () => {
const file = fileInput.files?.[0];
if (!file) return;
if (file.type !== "application/pdf") {
showError("Choose a PDF file.");
return;
}
const bytes = new Uint8Array(await file.arrayBuffer());
await openPdf(bytes);
});Do not send the file to a convenience endpoint for virus scanning, preview generation, or metadata extraction. If you need those functions, implement them locally or make the network behavior an explicit opt-in.
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Pass the byte array to getDocument and retain the returned loading task so the user can cancel a large or damaged file. Display progress from the task when available, and surface parsing errors as a file-specific message rather than a generic application failure.
let currentTask;
async function openPdf(data) {
currentTask?.destroy();
currentTask = pdfjsLib.getDocument({
data,
useWasm: true,
wasmUrl: "/assets/pdf.wasm",
useWorkerFetch: true
});
currentTask.onProgress = ({ loaded, total }) => {
if (total) updateProgress(loaded / total);
};
const document = await currentTask.promise;
renderPage(document, 1);
}Destroy the previous task when the user chooses another file. Otherwise old workers, decoded images, and page proxies can remain reachable and inflate memory use.
4. Render only what the user can see
To render a page, obtain a page proxy, choose a viewport at the display scale, and draw into a canvas. A production viewer should virtualize the page list: render the visible pages first, retain low-resolution thumbnails for nearby pages, and release canvases that are far outside the viewport.
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async function renderPage(pdf, pageNumber) {
const page = await pdf.getPage(pageNumber);
const viewport = page.getViewport({ scale: 1.25 });
const canvas = document.createElement("canvas");
canvas.width = viewport.width;
canvas.height = viewport.height;
const context = canvas.getContext("2d", { alpha: false });
await page.render({ canvasContext: context, viewport }).promise;
preview.append(canvas);
}Rendering a 500-page document at full resolution is a memory-management problem, not a test of parsing speed. Limit concurrent renders, reuse canvases where practical, and provide a cancel button for expensive operations.
5. Separate reading features from writing features
Build the first milestone around operations PDF.js exposes directly: page viewing, page count, metadata, text extraction, zoom, rotation in the viewer, and annotation display. For destructive edits or export, introduce a writer that can assemble a new PDF from selected pages. Keep that writer in a worker as well if it performs compression, image encoding, or object-stream reconstruction.
When merging or splitting, represent the user’s choices as a small operation plan rather than repeatedly copying whole documents in the interface thread:
const plan = [
{ source: "a.pdf", pages: [1, 3, 4] },
{ source: "b.pdf", pages: [2] }
];The plan can be validated before any expensive write begins. If an operation fails, the original files remain untouched and the user can retry with a narrower selection.
6. Return results as downloadable blobs
Once the writer produces bytes, create an object URL and trigger a download. Revoke the URL after the download link is no longer needed. This keeps the generated file in browser memory instead of placing it on a server.
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A worker improves responsiveness by moving parsing, decoding, and export away from the event loop that paints the interface. It does not make an operation free: the browser still needs CPU time and memory, and messages between the main thread and worker may copy large buffers.
Use transferable ArrayBuffer objects when passing ownership is safe. A transferred buffer is no longer usable by the sender, so transfer only after the interface has finished with the source data. For repeated previews, send compact page commands rather than full document bytes.
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WebAssembly is valuable when profiling identifies a stable, CPU-heavy routine such as image decoding, compression, cryptographic hashing, or a PDF-writing primitive. Keep JavaScript for orchestration and feature logic. Loading several large modules at startup can increase time-to-first-page, so lazy-load an editing module when the user opens an export tool.
Proving that files do not leave the browser
Privacy claims require an observable test plan. Run the application in a clean browser profile and inspect the Network panel while opening, rendering, editing, and downloading a PDF.
- Record every request made after the page loads, including requests triggered only after a file is selected.
- Check request methods, payload sizes, and initiators. A document upload may be hidden inside a telemetry or error-reporting request.
- Disable analytics and crash-reporting integrations, then repeat the test. Confirm that no PDF-derived metadata or stack payload contains file content.
- Inspect service-worker code and Cache Storage. Do not cache user-selected bytes unless the product explicitly explains the retention policy.
- Check IndexedDB, localStorage, and the Origin Private File System for document persistence after closing the tab.
- Use browser developer tools to verify that downloads are generated from a blob URL rather than a server response.
Also review third-party scripts. A page can have a local PDF parser and still leak information through an unrelated script that can read application state. A strict content-security policy, dependency pinning, and a documented data-flow diagram make the guarantee easier to maintain.
Remote PDFs, same-origin policy, and range requests
Local files and remote URLs follow different rules. PDF.js states that the browser’s same-origin policy blocks cross-origin requests by default. A remote document therefore needs a same-origin URL, appropriate CORS headers, or a server-side proxy. A proxy changes the privacy model because bytes now pass through that service.
For large remote PDFs, HTTP Range Requests can allow the viewer to fetch only the portions needed for display instead of downloading the complete file first. The server must support byte ranges and return the headers PDF.js expects. Range loading is useful for public documents, but it is not a way to make a private local workflow more private; it introduces a network dependency.
Compatibility, memory, and startup trade-offs
| Decision | Benefit | Cost or constraint |
|---|---|---|
| Local file processing | Document bytes need not be sent to a processing service; upload latency is removed. | Large files consume the user’s RAM and CPU, and device limitations become the user’s problem. |
| WebAssembly-assisted paths | Can speed selected routines such as image decoding. | Module download and compilation add startup work; not every feature runs in WASM. |
| Worker execution | Keeps parsing and rendering from blocking interface interaction. | Workers still consume memory, and message transfers can copy or transfer buffers. |
| Remote upload processing | Can provide server-scale CPU and centralized conversion. | Requires network transfer, server retention controls, and trust in the service. |
| Remote range loading | Displays portions of a large public PDF without fetching it all at once. | Requires same-origin access or CORS and a server that supports HTTP ranges. |
PDF.js modern builds target current browsers; its FAQ lists legacy support including Chrome 125 and newer and Safari 18 and newer, while noting Safari defects or missing features. Treat those numbers as the compatibility baseline for the cited build, not a promise that every browser feature behaves identically.
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There is no published benchmark for this particular suite. Measure on representative devices using page count, file size, image density, peak memory, time to first page, and export duration. A fast desktop result does not predict performance on a phone with limited memory.
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Common failures and recovery steps
“Setting up fake worker failed”
Check that the page is served over HTTP(S), that GlobalWorkerOptions.workerSrc points to an existing module, and that the server returns JavaScript with a usable MIME type. Do not test the worker by double-clicking index.html; file:// is specifically unsupported for the normal worker path.
The WASM file is blocked or ignored
Confirm the wasmUrl path, response status, content type, and content-security policy. If the module fails, log the fallback state and keep a JavaScript path for supported operations. Do not claim WASM acceleration unless the module actually loads in the target browsers.
A remote document fails while local files work
Inspect the response for CORS headers, redirects to another origin, authentication requirements, and range support. Test the URL with a same-origin copy before adding a proxy, because a proxy changes where document bytes travel.
The tab crashes on a large or image-heavy PDF
Lower the render scale, virtualize pages, cap concurrent renders, release unused canvases, and process export in chunks. Give users a clear size or page-count warning, but avoid a universal limit: available memory varies by device and browser.
Safari behaves differently
Test worker startup, WebAssembly loading, canvas limits, text selection, and annotation behavior separately. The PDF.js FAQ notes Safari defects or missing features, so provide a supported-browser message and a download fallback for workflows that depend on a browser-specific capability.
A release checklist for a genuinely client-side suite
- All document operations accept browser-held bytes and have no implicit upload endpoint.
- PDF parsing and expensive writing run in workers, with cancellation and cleanup.
- WASM usage is limited to documented, measured routines; JavaScript fallbacks are intentional.
- Worker, WASM, and application assets are version-pinned and served from controlled origins.
- Network, telemetry, service-worker, IndexedDB, and cache behavior has been inspected during real operations.
- Large documents are tested for peak memory and recovery from cancellation or failure.
- Remote URL support is clearly separated from local-file mode and documents its CORS and range requirements.
- Browser support is stated for the exact PDF.js build, with Safari-specific limitations tested.
Bottom line
The most credible zero-cloud PDF suite is a hybrid: PDF.js provides standards-based parsing and rendering, a Web Worker protects interface responsiveness, and WebAssembly accelerates only the routines that benefit from it. Local processing can remove upload latency and reduce exposure, but it does not eliminate browser memory limits or prove privacy by itself. Publish the network and persistence audit alongside the feature so “client-side” describes a verifiable data flow rather than a marketing shortcut.
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