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Yes—a PDF toolkit can process a file in the browser without uploading its contents, if the application passes the selected file’s bytes to browser-side PDF code and does not send them elsewhere. A Content Security Policy (CSP) can restrict important resource and connection channels, but it cannot by itself prove that an application never transmits file data. Treat local processing as an implementation to verify, not a privacy guarantee implied by a product label.
What “local PDF processing” means
In a local-file workflow, the user selects a PDF and the application supplies its bytes directly to code running in the browser. Mozilla’s PDF.js FAQ documents loading a PDF from raw binary data as a Uint8Array and recommends that approach over converting the file to base64, which uses more memory. See the PDF.js FAQ.
This describes a way to build an application; it does not establish how any particular deployed toolkit works. A tool could process locally, send data to a server, or combine local and remote features. Verify the implementation and observe its network activity before relying on it for sensitive documents.
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| Workflow | Where the PDF bytes come from | Network and browser considerations |
|---|---|---|
| File selected on the device | The browser provides the selected file’s bytes to the application’s local code. | Processing can remain in the browser, but application code may still make network requests. Review the code and the permitted destinations. |
| PDF fetched from a remote URL | The browser requests the PDF from a server. | This contacts a server. Under ordinary browser permissions, PDF.js says cross-origin requests are not available by default; CORS permission or a server proxy may be used. A remote-fetch workflow is not equivalent to processing a locally selected file. |
The cross-origin behavior and PDF.js details are documented in the PDF.js FAQ. A proxy changes where the browser connects, but it does not make a remote-PDF workflow a local-only one.
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What CSP can—and cannot—enforce
CSP is a browser-enforced policy that limits which resources a page may load and which script-driven connections it may make. The World Wide Web Consortium (W3C) says: “The Content-Security-Policy HTTP response header field is the preferred mechanism for delivering a policy from a server to a client.” Deliver the policy with the unprefixed Content-Security-Policy HTTP response header; the W3C Content Security Policy Level 3 specification describes this preferred mechanism and policy enforcement.
connect-src limits script-driven connections
The connect-src directive governs destinations used by script-driven connections, including fetch and XMLHttpRequest, WebSocket, EventSource, and beacon. Review its allowed origins against every connection the application needs. A strict local-only design should assess whether connections can be disallowed and whether features such as a reporting endpoint require an exception. Every exception expands the set of destinations the page is allowed to contact.
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worker-src controls worker script sources
PDF processing may use a Web Worker. The worker-src directive controls the URLs from which Worker, SharedWorker, or ServiceWorker scripts may load. Set it to the actual worker source required by the deployment, often a same-origin asset. If worker-src is absent, its policy falls back through child-src, then script-src, then default-src. See MDN’s documentation for worker-src.
Other directives still matter
connect-src is not a universal network firewall. Review the rest of the policy—including script, style, image, form, and frame directives—in light of the application’s behavior. MDN warns that 'unsafe-inline' and 'unsafe-eval' weaken protections for inline code and dynamic evaluation; see its Content-Security-Policy header reference.
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A policy narrows defined browser resource and connection channels; it does not certify that application code is benign or that every route for sending data has been considered. That is why CSP is defense in depth, not a substitute for inspecting the code, allowed origins and endpoints, and actual network behavior.
How to assess a browser PDF toolkit
- Establish the file path. Determine whether the tool uses a locally selected file or fetches a PDF from a URL. For a local-file implementation using PDF.js, check whether it passes raw bytes such as a
Uint8Arrayto browser-side code. - Inspect the production CSP. Read the actual
Content-Security-Policyresponse header. Checkconnect-srcfor permitted connection destinations andworker-src—including its fallback chain—for the PDF worker source. - Review the rest of the application’s routes. Inspect relevant directives and code paths, allowed origins and endpoints, and observable network activity while opening and processing a document. A policy exception or an unreviewed route can change the privacy boundary.
- Test in the actual target environment. Check the production response headers and the browser builds the application supports. An overly restrictive policy can block a needed worker or asset; a permissive exception allows a broader set of resources or connections.
CSP can also be delivered in report-only mode while a policy is being tuned, but report-only monitoring does not constrain browser behavior. An enforcing policy is needed to restrict what the browser permits. The W3C specification covers both enforcing and report-only policies in its CSP Level 3 documentation.
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PDF.js compatibility and worker versions
Browser feature support varies, according to the PDF.js FAQ. Its modern-build table lists Firefox and Chrome as supported and describes automated testing on Windows and Linux. Its legacy-build table lists Firefox ESR+, Chrome 125+, Opera, Chromium-based Edge, and Safari 18+ as “Mostly” supported. These are the FAQ’s stated compatibility details, not a guarantee for every later release or configuration; check the project’s current FAQ and the version being deployed.
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