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Compress PDF

Shrink your PDF by losslessly optimizing its internal structure, no image quality loss. The result is local; nothing is sent to a server.

How Compress PDF works

PDF Compress reduces the file size of a PDF through lossless structural optimisation: it rebuilds the cross-reference table, merges duplicate objects, removes obsolete update sections left by previous saves, and packs content into compressed object streams. All of this runs in pdf-lib inside your browser. Your PDF is never sent to a server or cloud service.

Two modes, and the difference matters. The default is that lossless pass: it never downsamples, re-encodes or strips an embedded image, so the document you get back is the one you put in, only packed better. Strong mode is the opposite bargain: each page is re-rendered as a JPEG image at a DPI you choose, which shrinks a scanned PDF dramatically but produces an image-only file. The text stops being selectable or searchable, and links and form fields do not survive. Pick it deliberately, on a document where size beats everything else. If your PDF is large primarily because it contains high-resolution photographs, the savings from structural cleanup alone will be modest. The biggest gains come from PDFs that have accumulated incremental updates over multiple saves, contain duplicate resource objects, or have uncompressed cross-reference sections. For image-heavy PDFs where size matters, combining this tool with image compression before embedding is the more effective path.

Written by Bastien Sulyan

How to use Compress PDF, step by step

  1. Load your PDF into the compression tool.
  2. Review the original file size shown in the interface.
  3. Choose the mode: lossless keeps the document intact, strong re-renders each page as an image for a much smaller file.
  4. Click compress to run the optimisation pass.
  5. Download the output and compare its size to the original.

Common use cases

  • A form PDF has been opened and saved dozens of times, accumulating incremental update sections; structural cleanup recovers significant space.
  • A generated report has duplicate font or image resource objects embedded multiple times; deduplication reduces the file.
  • An email attachment is slightly over the server size limit; lossless optimisation brings it under the threshold without touching image quality.
  • A legal document bundle needs to be archived; removing redundant internal structures makes the archive more compact.

Frequently asked questions

Will compressing my PDF reduce image quality?

In the default lossless mode, no: the structural pass does not touch the pixel data of any embedded image, so photos, scans and logos come out at exactly the resolution and quality they went in. In strong mode, yes, and by design: every page is re-rendered and re-encoded as a JPEG at the DPI and quality you set, so image quality drops and the page text stops being selectable. The mode selector says so before you run it.

Why did my PDF barely shrink?

Structural optimisation has diminishing returns on PDFs that are already well-formed. If the bulk of your file size comes from high-resolution images and the document has never been edited in a way that left redundant objects, the savings will be small. In that case, you would need to re-export the document with lower image resolution from the source application.

What are incremental update sections and why do they waste space?

Each time a PDF is saved after editing, many viewers append the changes rather than rewriting the whole file. Over many cycles this can leave large amounts of superseded data. Structural compression rewrites the file from scratch, discarding all those obsolete layers.

Does my document leave my device during compression?

No. pdf-lib runs entirely in your browser tab. The file is loaded into memory in your browser, restructured there, and the output is assembled in memory and handed to you as a local download. No copy of your PDF travels over the internet.

Does this tool remove bookmarks or annotations to save space?

No. Only redundant structural overhead is removed. Bookmarks, annotations, form fields and all page content are preserved intact in the output.

Can I compress a PDF that already has a password?

Encrypted PDFs can be complex to rewrite without the owner password because the internal structure is protected. If compression does not produce the expected result on an encrypted file, try using the PDF Unlock tool to remove the owner restrictions first, then compress.

Will compressing my PDF add a watermark?

No. The output contains the same content restructured for size, with no watermark, logo or branding stamped anywhere. Some free online compressors add a promotional mark on the output; this tool never does.

Does PDF Compress work on a phone browser?

Yes. The whole optimisation runs in plain JavaScript, so it works on mobile Chrome, Safari and Firefox the same as on desktop. Loading a very large PDF, several hundred megabytes, into a phone browser tab is the main practical limit, not the compression logic itself.