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What “client-side” and “zero uploads” mean
In a client-side workflow, the browser reads a file selected from the user’s device and performs the PDF work locally. The design can avoid sending those document bytes to a PDF-processing backend. “Zero cloud uploads” should be limited to that claim: it does not automatically mean the page makes no network requests, or establish what happens to analytics, telemetry, third-party scripts, OCR, or crash reports. Those behaviors depend on the actual application and its deployment.
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The available library documentation supports the feasibility of browser-based PDF processing; it does not verify the existence, feature set, or traffic behavior of any particular toolkit. A builder should describe the implementation and substantiate its privacy claim with source-code review and runtime checks.
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PDF.js separates its work into three layers: a core layer that parses and interprets PDF files, a display layer that exposes rendering and document information, and a viewer layer that provides user-interface components. The prebuilt distribution includes pdf.mjs and a separate pdf.worker.mjs, as well as supporting assets. See the PDF.js getting-started guide.
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If you start from PDF.js’s generic viewer, Mozilla asks integrators to re-skin it or build upon it rather than embed it unchanged. A custom interface can provide the controls your application needs—such as page navigation, zoom, or a file list—while the library handles parsing and rendering.
Run development through a local web server
PDF.js documents that its worker is not enabled when the application is opened through a file:// URL. Serve the app through a local web server during development instead; a local server is a development setup, not evidence that user documents are uploaded to a remote service. Follow the project’s setup guidance.
Keep the worker and library on the same release
The PDF.js API bundle and worker must use matching versions. The project’s FAQ on API and worker version mismatches notes that different versions are not guaranteed to work; an outdated cached worker or files loaded from different versioned locations can produce a version error. If that happens, check both file paths and clear stale cached assets before changing unrelated code.
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Use pdf-lib for editing and generation
PDF.js is suited to parsing and rendering; editing and creating documents can be handled separately with pdf-lib. The project describes the library as pure JavaScript without native dependencies and documents browser use. Its listed capabilities include creating and modifying PDFs, drawing text and images, embedding fonts and pages, adding or removing pages, splitting and merging documents, and reading or filling common form fields. These are library capabilities, not proof that a particular application implements every feature or supports every PDF reliably.
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The pdf-lib homepage says it “Works in any JavaScript runtime, including browsers, Node, Deno, and even React Native.” That is the project’s own compatibility description, not an independent test of every runtime or browser.
Treat PDF-open JavaScript as a distinct capability
pdf-lib also exposes an API for adding JavaScript that runs when a PDF is opened. That is different from ordinary page editing or drawing. If an application creates or handles such actions, explain the behavior and consider its security implications; the existence of a library API does not mean active content is automatically neutralized. See the pdf-lib API documentation for addJavaScript.
Read file bytes directly and account for browser resources
For a local file workflow, pass PDF data as a Uint8Array rather than converting it to base64. The PDF.js FAQ warns that base64 conversion uses more memory and recommends typed-array data. This avoids an unnecessary representation change; it does not establish a universal file-size limit or guarantee a particular processing speed.
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PDF performance varies with the document, the device, browser behavior, and implementation choices. PDF.js cautions against rendering every page at high resolution and notes that some PDFs are slow. Render pages as the user needs them when appropriate, and test the kinds of documents and devices your application is intended to support. The documentation provides no universal maximum size or benchmark that can be applied to every toolkit.
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Handle remote URLs as a separate input path
Loading a PDF from a user-selected local file is different from fetching one from a remote URL. Under ordinary JavaScript permissions, PDF.js says cross-origin requests are not allowed by default; remote access may require the file server to permit CORS or a server proxy. See the PDF.js FAQ on cross-origin requests.
If the privacy promise is that selected documents are not uploaded for processing, keep local-file processing distinct from any optional remote-URL feature. A proxy changes where a remote fetch is made and should be explained separately; the documentation does not establish how any particular application handles such requests.
Verify the no-upload claim on the actual site
Library choice alone cannot establish that a deployed application sends no document bytes to a server. Verify both the implementation and its runtime behavior before making that claim.
- Review the file-processing path. Trace how a selected file is read, passed to PDF.js or pdf-lib, and turned into a download. Look for application code that transmits document data to a server or invokes remote processing.
- Inspect network activity while using the app. In browser developer tools, observe requests during file selection, processing, and download. Check whether document bytes or derived page data are sent, and identify other requests separately rather than treating every request as a document upload.
- Check loaded scripts and services. Review external scripts and integrations alongside the app’s own code. A local PDF-processing path does not establish the behavior of unrelated analytics, telemetry, OCR, or crash-reporting services.
- Repeat against the deployed build. Confirm that the production configuration, hosted assets, and worker files match the implementation you reviewed. Document the scope of the check and the tested browser and version; PDF.js browser support is not identical across all browsers and versions.
Until those checks establish the behavior of the specific deployment, describe the design as intended to process selected files locally rather than asserting that the site is independently verified to make zero uploads.
Keep compatibility claims specific
PDF.js publishes a browser-support table with qualifications for some entries. Do not turn that table into a blanket claim that every browser or version works. Test the exact browser and version combinations your application supports, including PDF display, worker loading, editing, and export; report only combinations actually checked in that project. The PDF.js browser-support FAQ is a starting point, not a substitute for application-specific verification.
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