How PDF Compression Works
PDF compressors mainly shrink the images inside a file: they lower resolution (downsampling) and re-save them at lower quality. They also remove unused data and duplicate objects. Text-only PDFs are already small and barely shrink.
Where PDF file size comes from
In a typical oversized PDF, images account for 80–95% of the size. The rest comes from fonts, page content, metadata and sometimes attachments or leftover editing history.
Lossless compression (no quality loss)
- Flate (ZIP) compression of page content and data.
- Object streams: packing many small objects together (PDF 1.5+).
- Removing unused objects, duplicate images, thumbnails, metadata, old revisions from incremental saves.
- Font subsetting: keeping only the characters used.
Typical saving: 5–30%.
Lossy compression (smaller, some quality loss)
- Downsampling: reducing image resolution, e.g. from 600 DPI to 150 DPI. Halving DPI cuts pixel count to a quarter.
- JPEG re-compression at lower quality.
- Colour conversion: turning colour scans into greyscale or black-and-white.
- Better codecs: JBIG2 for black-and-white scans.
Typical saving on image-heavy files: 50–90%.
Choosing a compression level
| Level | Image resolution | Good for |
|---|---|---|
| Low / “high quality” | ~300 DPI | Printing |
| Medium / “recommended” | ~150 DPI | Email, uploads, on-screen reading |
| High / “extreme” | 72–100 DPI | Hitting strict size limits; may look blurry when zoomed |
What compression can’t do
- It can’t make a text-only PDF dramatically smaller.
- It can’t restore quality once it’s lost. Always keep your original.
- Very aggressive compression can make small print or signatures unreadable. Check the result before sending.
Ready to shrink a file? Follow How to compress a PDF.
Frequently asked questions
Does compressing a PDF reduce quality?
Lossless steps (removing unused data, optimising structure) don't. Lossy steps (downsampling and recompressing images) do, but at sensible settings like 150 DPI the difference is rarely visible on screen.
Why did compressing my PDF barely change its size?
The file is probably mostly text and vectors, which are already compact, or it was already compressed. Large embedded fonts or attachments can also be the culprit.
Can I compress a PDF without uploading it?
Yes. macOS Preview, Adobe Acrobat desktop, LibreOffice and the free Ghostscript tool all compress PDFs locally on your computer.
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