Compress PDF Online Free

PDF compression reduces file size by rewriting the document with more efficient encoding. It matters when files exceed email limits, when a portal caps upload size at 5MB, or when a client's inbox groans at a 40MB attachment. Compression is not lossless in every case: images may be resampled, but text and vector content usually stay pixel-perfect. The tool offers three levels — low, medium, high — that trade file size against fidelity. Low preserves visual quality and typically shrinks files 15-35 percent by rewriting internal structure only. High resamples images and may reduce photos to 100 DPI, shrinking image-heavy files up to 70 percent at the cost of some sharpness on zoom. Everything runs in your browser using pdf-lib. Nothing is uploaded. The output is a standard PDF that opens in every reader — no proprietary format, no cloud dependency.

Last reviewed: July 2026

What is this?

PDF compression works through three mechanisms that operate independently. First, image resampling downscales embedded photos to a lower DPI, which is the biggest lever for files dominated by scans or screenshots. Second, font subsetting removes glyph data for characters the document does not use — a 300KB font can shrink to 20KB if only 200 glyphs are actually referenced. Third, object stream compression bundles small internal objects into deflate-encoded streams, reducing overhead in file structure. A well-optimized PDF has already used these; the tool's biggest wins come on files exported from Word, LibreOffice, or scanning apps that skip these steps.

How to use this tool

1. 1Choose the PDF to compress. Any size works, but files above 100MB may strain browser memory. Look at the file size before proceeding — some already-optimized PDFs will not shrink much.
2. 2Pick a compression level. Low keeps images sharp and reduces size 15-35 percent. Medium is a balanced default. High aggressively resamples images and can cut file size up to 70 percent, but photos may look softer.
3. 3Click Process. The tool reads the PDF, applies the chosen strategy, and rewrites the document. Larger files may take 10-30 seconds in the browser — a progress bar shows real-time status.
4. 4Download the compressed PDF and compare it with the original. If the new file is not much smaller, the source was already optimized, and going higher will hurt image quality without meaningful size gains.

Tips & best practices

• Compression ratios vary wildly: PDFs with lots of embedded photos may shrink 60-80 percent, while text-only PDFs typically only drop 5-15 percent.
• Do not run compression twice. The second pass gives negligible improvement and only hurts image quality further.
• For files headed to print, use Low. For screen viewing (email, web), Medium works. Reserve High for PDFs where the recipient does not need to zoom.
• If your target is a specific size (say 200KB), start with Medium and iterate — there is no way to guarantee an exact output size, only ranges.
• Compressing scanned documents made from mobile photos can produce very small files but may make text unreadable at zoom levels above 200%. Test the output before sending it to a printer.