How does PDF compression work here?

Compression runs in three stages: first, zero-length and orphaned streams are pruned. Then all content streams are deflated using zlib. Finally, PDF objects are packed into compressed object streams, which is a PDF 1.5+ technique that significantly reduces cross-reference table overhead. Optionally, JPEG images embedded in the PDF can be re-encoded at a lower quality level for additional size reduction.

Why did the file size not change?

Some PDFs are already compressed at the stream level or contain mostly binary data like images and fonts that cannot be deflated further. If the output is not smaller than the input, a warning is shown.

Is this lossless? Will anything look different?

By default, yes. The structural compression (zlib + object streams) is fully lossless: no images are re-encoded, no fonts are subsetted, and no content is removed. If you choose an image quality setting below Full, JPEG images in the PDF are re-encoded at that quality level. This is lossy: images may look slightly softer, and the output is not bit-for-bit identical to the original.

Are my files uploaded to any server?

No. Compression runs entirely in your browser using a Rust library compiled to WebAssembly. Your files never leave your device.

Is there a file size limit?

No fixed limit. Processing happens in your browser tab using available device memory. Most documents, including large ones, process in under a second.

Which PDF versions are supported?

The tool reads any PDF version and writes using object streams, which are a PDF 1.5+ feature. PDF 1.5 has been supported by all major viewers (Adobe, Preview, Chrome, Firefox) since 2005.

Can I compress a password-protected PDF?

Encrypted PDFs cannot be compressed without the password. Use the PDF Password tool to unlock the file first, then compress it.

What is the difference between Light, Standard, and Maximum compression?

These control the zlib compression level applied to object streams. Light (level 1) compresses quickly with moderate size reduction. Standard (level 6) is the default and works well for most files. Maximum (level 9) produces the smallest files but takes longer, noticeable on very large PDFs.

What does the image quality setting do?

PDF files often embed JPEG photos as their dominant weight. The image quality setting re-encodes those JPEG images at a lower quality to reduce their size. Full preserves images exactly as they are. 85%, 70%, and 50% re-encode at that JPEG quality level: lower values produce smaller files but softer images. Only JPEG images without transparency are affected; other image types are left untouched.

Also try the PDF Editor

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Drop a PDF and re;file labs compresses it. Lossless structure compression runs by default. Optionally re-encode embedded JPEG images at lower quality for maximum size reduction. No uploads, no data sent to external servers.

Compression

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How it works

Lossless compression plus optional image recompression

First, empty and unreferenced streams are pruned. Next, all content streams are deflated using zlib. Finally, PDF objects are packed into compressed object streams (a PDF 1.5 feature), removing cross-reference table overhead. These three stages are fully lossless. If you choose an image quality below Full, embedded JPEG images are additionally re-encoded at that quality for extra size reduction. Everything runs in your browser via WebAssembly.

Use cases

When to compress a PDF

Email attachments

Shrink PDFs before attaching them to emails. Most providers cap attachments at 10–25 MB. Compression can make the difference.

Cloud storage

Reduce storage consumption when archiving large document collections in Google Drive, Dropbox, or OneDrive.

Mobile-friendly docs

Smaller PDFs load faster and use less data when opened on phones and tablets, especially on slow connections.

Long-term archiving

Compress PDFs before adding them to archives. Use lossless mode to preserve everything exactly, or reduce image quality for smaller archive footprints.

FAQ

Common questions

Everything you need to know about PDF compression.