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What this workflow does—and what it does not
PDF.js is a parsing and rendering component. It loads a PDF, exposes document and page objects, and can render pages to a canvas. It is not, in the documented workflow described here, the step that writes a compressed replacement PDF.
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The output step is reconstruction: encode each rendered page as an image, create a new PDF, and place one image on each new page. pdf-lib documents creating PDF documents, embedding JPEG or PNG image bytes, adding pages, and saving in a browser context. The resulting pages are image-based representations of the originals, not the original PDF objects with their data selectively optimized.
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This can reduce size for some scan-heavy or image-heavy files, depending on the source and chosen resolution and image encoding. It can also make a file larger. It generally sacrifices the source document’s text and vector structure, and may not preserve links, forms, or other interactive features in the same way. Treat it as a trade-off, not a universal “compress” button.
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How the pieces fit together
| Component | Role in the workflow | Important limit |
|---|---|---|
| PDF.js | Loads the selected PDF, retrieves pages, and renders them using a viewport and canvas context. | It supplies parsing and rendering, not the reconstructed output PDF. |
| OffscreenCanvas | Can assist image conversion or rendering work where supported. | It is a canvas API, not a PDF-writing format or compression engine. |
| Web Worker | PDF.js distributions include a separate core worker; worker-thread rendering is an optional capability in the draft API documentation. | A worker’s presence does not prove all rendering occurs off the UI thread. Rendering can fall back to the main thread. |
| PDF output library | Embeds encoded page images into newly created PDF pages and saves the result. | Embedding raster images does not restore the source’s text, vectors, or interactive structure. |
Build the page-by-page pipeline
- Accept a local file. Let the user choose a PDF and read its bytes in the browser. Passing a locally selected file avoids the cross-origin fetch dependency that applies when loading a PDF from another website.
- Load and inspect with PDF.js. Use the installed PDF.js display library to load the bytes and retrieve the document. Its API is asynchronous; plan for loading and page operations to complete asynchronously rather than blocking the interface.
- Choose a rendering scale. For each page, create a viewport at the selected scale. Set the canvas dimensions from that viewport, accounting for the desired output scale, and render the page using the canvas context and viewport. Higher dimensions preserve more fine detail but require more memory and can produce larger encoded images.
- Encode the rendered page. Convert the canvas output to an image format and quality chosen by the implementation. JPEG is a practical option when lossy image encoding is acceptable; PNG is also supported by pdf-lib but has different size and fidelity trade-offs. Do not promise a particular quality level or byte reduction without testing the actual file and settings.
- Create and save a replacement PDF. Use a PDF-writing library to create a document, add a page for each rendered source page, embed that page’s image, and save the output in the browser. Preserve page proportions when sizing the image on the output page so the reconstructed page is not stretched.
Keep the PDF.js display library and its worker assets on the same version. PDF.js warns that mismatched versions are not guaranteed to work and can produce an API/worker version error. Bundle or serve the worker through the deployment setup for the pinned version; PDF.js documentation says its worker is not enabled for file:// URLs, so test through the local or production server path you intend to use.
Use OffscreenCanvas and workers without assuming they solve everything
OffscreenCanvas is best understood as an optional rendering or image-conversion aid. PDF.js documentation describes worker OffscreenCanvas support as primarily intended to improve image conversion and rendering performance, and documents a canvas-area threshold used to decide when an image is resized through OffscreenCanvas. That does not make OffscreenCanvas responsible for writing the output PDF.
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There are two separate questions when using workers: whether PDF.js’s core worker is configured, and whether page rendering itself runs in a worker thread. The draft API documentation describes optional rendering in a worker thread that requires GlobalWorkerOptions.rendererSrc. If that renderer worker is absent or fails, rendering falls back to the main thread; a custom ownerDocument also disables worker rendering. Because this API documentation is explicitly a draft and may change, confirm the configuration against the PDF.js version you pin.
- Check worker startup in the deployed build rather than inferring it from a successful local render.
- Keep a fallback path for main-thread rendering, and present progress or cancellation controls if large documents could keep the interface busy.
- Test the exact production asset paths and browser support for the versions you ship.
Control memory by rendering incrementally
Canvas memory can become substantial before image encoding or PDF assembly is considered. Mozilla PDF.js’s FAQ estimates that one letter-size page rendered at 816 × 1056 pixels, with 4 bytes per pixel at 96 DPI, occupies about 3,446,784 bytes—approximately 3.5 MB. The same FAQ estimates about 14 MB per page at device pixel ratio 2. These are illustrative canvas-memory estimates, not measurements of a compressor.
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At those settings, a 100-page document can require roughly 3.5 MB per page’s canvas memory at 96 DPI, or about 14 MB per page at device pixel ratio 2, before accounting for other work. Avoid keeping every full-size canvas alive at once: render, encode, add the image to the output document, then release or reuse canvas resources as appropriate. This is an engineering precaution, not a measured performance result for a particular implementation.
Offer settings that expose the real trade-off
Give users control over the factors that determine output quality and size instead of presenting an unexplained compression percentage. A useful interface can include:
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- Resolution or scale: lower settings reduce the pixels available for fine print and detail; higher settings use more memory and may create larger images.
- Image format and quality: explain that lossy encoding can reduce image detail, while other formats may produce larger output depending on page content.
- Output comparison: show the source and generated file sizes after processing, rather than predicting savings in advance.
- Feature warning: tell users before conversion that output pages are rasterized and may not retain selectable/searchable text, links, forms, vectors, or document structure as in the source.
There is no source-published universal compression ratio for this pipeline. Report measured results only for named test files and declared settings; do not generalize a result from one PDF to all documents.
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Evaluate representative documents against the original using the same settings. A smaller file is not automatically a better result if text becomes hard to read or important document behavior disappears.
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- Scanner type: Document
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- Digitize documents and images
- Compare output size with source size.
- Inspect small text, fine lines, diagrams, and photographs at normal viewing size and when zoomed.
- Check whether searchable text, links, forms, and vectors are still present if users need them.
- Record the chosen output resolution and image encoding quality alongside measured results.
- Observe processing time and peak memory on the page counts and devices you intend to support.
- Verify worker availability and fallback behavior in the deployed build, including the server setup used to serve the PDF.js assets.
Handle remote PDFs separately
If users provide a URL instead of selecting a local file, the browser’s normal cross-origin rules apply. The remote server must permit the browser request through suitable CORS behavior, or the application needs an appropriate proxy. A proxy changes the architecture and moves some handling outside the browser; it should not be described as a wholly browser-only file flow.
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