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Yes. A browser-based PDF toolkit can process a locally selected file without sending its bytes to a processing server, provided its code passes those bytes directly to browser-side PDF processing and does not transmit them elsewhere. A Content Security Policy (CSP) can restrict certain browser resource loads and script-driven connections, but it cannot by itself prove that a toolkit never uploads file data. That depends on the implementation and the policy’s permitted routes.
How local PDF processing works
A web app can read a file selected on the device and pass its bytes to PDF-processing code running in the browser. Mozilla’s PDF.js FAQ documents loading raw binary data as a Uint8Array and recommends this over converting the PDF to base64, which uses more memory. This describes a supported input method; it does not establish how any particular deployed toolkit handles files.
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In a local-file workflow, the selected file’s bytes are available to the browser application. Whether they stay there depends on what the application code does next. A toolkit might process the bytes locally, or it could send data to a server through a permitted network route. Confirm the implementation rather than inferring its behavior from the fact that it runs in a browser.
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What CSP can—and cannot—enforce
CSP is a browser-enforced policy that controls specified classes of resource loads and connections. The World Wide Web Consortium says, “The Content-Security-Policy HTTP response header field is the preferred mechanism for delivering a policy from a server to a client.” Use the unprefixed HTTP response header for an enforcing policy; a report-only policy can help monitor effects while a policy is being tuned, but does not constrain behavior in the same way.
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The directives that matter for a local PDF toolkit have distinct jobs:
connect-srclimits script-driven connections such as fetch, XMLHttpRequest, WebSocket, EventSource, and beacon. Review the permitted destinations and endpoints. A local-only design should assess whether connections can be disallowed and whether necessary application features or reporting endpoints require exceptions.worker-srccontrols the URLs from which Worker, SharedWorker, and ServiceWorker scripts may load. Set it to the worker source the deployment actually needs, often a same-origin asset. Ifworker-srcis absent, its policy falls back throughchild-src,script-src, and thendefault-src.- Other directives govern other resource types. Review script, style, image, form, frame, and other relevant loads as well;
connect-srcis not a universal network firewall. MDN warns thatunsafe-inlineandunsafe-evalweaken CSP protections for inline code and dynamic evaluation.
CSP narrows the browser’s allowed channels, but it is not a complete privacy audit or an attestation that application code is benign. A policy only helps with routes it covers. Review the application source, permitted origins and endpoints, relevant directives, and observable network activity before concluding that file data stays local.
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Local file versus remote PDF
| Workflow | Where the PDF bytes come from | Network and policy considerations |
|---|---|---|
| Locally selected file | The browser application receives bytes from a file selected on the device; it can pass them to PDF processing locally. | Local processing does not require fetching the PDF from another server. Inspect application code and permitted connections to establish whether the bytes are transmitted. |
| PDF loaded from a remote URL | The browser requests the PDF from the server hosting it. | This contacts another server. Under ordinary browser permissions, cross-origin requests are not available by default; the server may need to allow access through CORS, or the application may use a server proxy. This is not equivalent to processing a locally selected file. |
PDF.js explains its cross-origin limitations in its FAQ. A remote-URL feature should therefore be treated as a separate data flow: identify the server contacted and determine whether the request goes directly from the browser or through a proxy.
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What to verify in a PDF.js deployment
- Input path: Check whether a local file is passed to the PDF-processing code as bytes, rather than uploaded to a processing endpoint.
- Connections: Inspect
connect-src, its allowed origins, and the application’s actual network activity for destinations that could receive file data. - Worker source: Confirm that
worker-srcpermits the worker asset the deployment uses, and inspect the fallback directives if it is omitted. A policy that blocks the worker can break PDF processing. - Version matching: PDF.js requires its API and worker versions to match exactly.
- Browser support: PDF.js support varies by browser and build. Its FAQ lists Firefox and Chrome for the modern build and says automated testing is on Windows and Linux. Its legacy-build table lists Firefox ESR+, Chrome 125+, Opera, Chromium-based Edge, and Safari 18+ with support marked “Mostly.” Check the current PDF.js FAQ and the specific release before relying on those details.
- Production behavior: Evaluate the actual production response headers and browser builds. Do not treat a policy example as proof that a deployed toolkit has been tested.
How to interpret a “files stay in your browser” claim
That claim is credible only when the implementation and its permitted routes support it. A local input method shows that browser-side processing is possible; an enforcing CSP limits defined resource channels. Neither fact alone proves that the application never sends file data. For confidence, check the code path that handles the selected file, the policy’s directives and allowed destinations, and network activity during the relevant workflow.
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