How does in-browser PDF compression work?

Lossless compression restructures the PDF using object streams. All content stays identical but the file is packed more efficiently. Image-based modes re-render each page as a JPEG at the selected quality level, which can reduce file size significantly for PDFs with many images.

Will compression reduce PDF quality?

Lossless mode preserves quality exactly. High, Medium, and Low modes re-render pages as JPEG images, which removes selectable text in exchange for a smaller file, best for scanned documents.

How much can I reduce a PDF file size?

It depends on what the PDF holds. Lossless mode repacks the file and usually trims a modest amount from text-heavy PDFs. High, Medium, and Low re-render every page at 108, 86, or 65 dpi, so a scan stored at a higher resolution usually shrinks a lot, while a mostly-text PDF can shrink less or even grow. The tool tells you when the result is not smaller.

What resolution do High, Medium, and Low use?

High renders each page at 108 dpi with 85% JPEG quality, Medium at 86 dpi with 70%, and Low at 65 dpi with 55%. Each page keeps its original size; only the image drawn on it gets coarser.

What if the compressed file is larger than the original?

Some PDFs are already well-optimised and cannot be made smaller without quality loss. The tool will notify you if the compressed version is not smaller than the original.

Compress PDF Online

Reduce your PDF file size. Choose lossless optimisation to keep all content intact, or pick a lower quality level for maximum compression. Everything runs in your browser.

Drop a PDF here or click to browse

How it works

Lossless mode uses pdf-lib to re-serialise the document with object stream compression, packing the existing content more efficiently without altering it. The image-based quality modes hand each page to pdfjs-dist, which renders it onto a Canvas, exports the canvas as a JPEG, and lets pdf-lib embed those JPEGs into a fresh output document. Picking the right mode for your file is the fastest way to shave megabytes off.

Processing runs in your browser

Every step of the compression pipeline, parsing, page rendering, and writing the smaller file, runs inside this browser tab. pdf-lib and PDF.js execute as JavaScript loaded by the page, with no PDF data ever sent to our servers. You can check this yourself in your browser's DevTools Network tab.

Related operations

To stitch multiple PDFs into one before compressing, try merge PDF. For pulling out a subset of pages, use split PDF. To extract pages as standalone images, see PDF to image.

Frequently asked questions

How does in-browser PDF compression work?: Lossless compression restructures the PDF using object streams. All content stays identical but the file is packed more efficiently. Image-based modes re-render each page as a JPEG at the selected quality level, which can reduce file size significantly for PDFs with many images.
Will compression reduce PDF quality?: Lossless mode preserves quality exactly. High, Medium, and Low modes re-render pages as JPEG images, which removes selectable text in exchange for a smaller file, best for scanned documents.
How much can I reduce a PDF file size?: It depends on what the PDF holds. Lossless mode repacks the file and usually trims a modest amount from text-heavy PDFs. High, Medium, and Low re-render every page at 108, 86, or 65 dpi, so a scan stored at a higher resolution usually shrinks a lot, while a mostly-text PDF can shrink less or even grow. The tool tells you when the result is not smaller.
What resolution do High, Medium, and Low use?: High renders each page at 108 dpi with 85% JPEG quality, Medium at 86 dpi with 70%, and Low at 65 dpi with 55%. Each page keeps its original size; only the image drawn on it gets coarser.
What if the compressed file is larger than the original?: Some PDFs are already well-optimised and cannot be made smaller without quality loss. The tool will notify you if the compressed version is not smaller than the original.

Last reviewed September 30, 2026

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