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What Compress PDF does
Compress PDF shrinks file size by re-encoding the photographs embedded in a document at a lower quality and resolution, using your browser's own image encoder.
PDF file size is almost always photographs. A hundred pages of text might be under a megabyte; a single high-resolution scan can be ten. So compressing a PDF really means compressing the images inside it, and how much you gain depends entirely on how many there are.
This tool finds the JPEG images embedded in your document, redraws each one smaller and at reduced quality through a canvas, and swaps it back in — but only if the new version is genuinely smaller. Afterwards it tells you exactly how many images it recompressed, so you can tell the difference between "this worked" and "there was nothing here to compress".
How to use Compress PDF
Upload the PDF
Drop in your file. Its current size is shown so you have a baseline.
Pick a compression level
Low keeps images close to original quality; High shrinks hardest with visible quality loss. Medium is a reasonable default.
Compress
Click Compress. Each embedded photo is re-encoded on your device.
Check the result
The new size is shown next to the original, along with how many images were actually recompressed.
Compress PDF: details and limits
Effectiveness depends on how image-heavy the PDF is. A photo-heavy scan can shrink dramatically; a text-and-vector document like a contract has almost nothing to compress and may barely change. The tool says which happened rather than pretending.
Text and vector graphics are never touched, so they stay perfectly sharp at any compression level.
The three levels are specific rather than vague. Low re-encodes each image at JPEG quality 0.82 and caps its longest side at 2200 pixels; Medium uses 0.65 and 1600; High uses 0.42 and 1100. An image already smaller than the cap is never enlarged to meet it.
Only images stored as plain JPEG — /DCTDecode, in PDF terms — are re-encoded. Other encodings are left alone rather than risking a corrupted or worse-looking result, so a PDF full of PNG-style images can report zero recompressed and be working correctly.
A JPEG with a transparency mask, or in CMYK or another print colour space, is also left untouched — and counted, so the tool tells you. Re-encoding goes through your browser's canvas, which would flatten the transparency and mangle CMYK colours; we tested a magenta CMYK scan and it came back green, which is why those images are skipped instead.
An image is only replaced if the recompressed version is actually smaller — compression never makes part of your file bigger.
Compress PDF: frequently asked questions
Guides
- How-to · 2 min
How to reduce PDF file size
What actually makes a PDF smaller, why some files barely shrink at all, and how to compress one without wrecking the text.
- How-to · 4 min
How to compress a PDF in Adobe Acrobat — and free, without it
Compress a PDF the way Acrobat does — but free and without uploading it. Pick a level; it re-encodes the embedded photos in your browser and tells you how many it changed.
- How-to · 3 min
How to compress a PDF without losing quality
No PDF compressor shrinks photographs without some quality loss — re-encoding a JPEG is lossy by nature. What stays lossless is the text and vector graphics, which are never touched. Pick Low for near-invisible loss.
- Article · 2 min
Why your PDF is 40MB
Almost always photographs. A look at what is actually inside a bloated PDF, and why a text-heavy file will not compress.
- Article · 3 min
Is it safe to upload a PDF to an online tool?
Usually nothing goes wrong — but you are handing a copy of your file to a stranger's server. How to judge the real risk, and when not to take it at all.
- Article · 3 min
Do online PDF tools really delete your files?
Most say they delete uploads within an hour. That is usually sincere, entirely unverifiable, and quietly leaves backups and logs out of the promise.