How PDF compression works
Most of the weight of a PDF comes from images: photos from a phone or a scanner are often stored at 300–600 ppi, while a screen or an office printer needs far less. The PDF Compressor measures how large every image is actually displayed on the page and downsamples it to the resolution of the level you choose, then re-encodes it with MozJPEG, a highly efficient JPEG encoder.
The rest of the file is optimized without any quality loss: duplicate images and fonts are merged, unused objects are dropped, content streams are recompressed and small objects are packed into compressed object streams. Page thumbnails and hidden editor data (for example from Illustrator or InDesign) are removed.
Text and vector graphics are never rasterized, so the result stays sharp at every zoom level and remains searchable. Only if you enable “Flatten pages” are pages turned into images — useful for photo books or when a very small target size can’t be reached otherwise.
Which compression level should I use?
| Level | Images | Best for |
|---|---|---|
| Lossless | unchanged | Contracts, legal and archive copies |
| High quality | 220 ppi | Printing, portfolios, brochures |
| Balanced (recommended) | 150 ppi | Email, sharing, most documents |
| Strong | 110 ppi | Upload portals with strict limits |
| Extreme | 72 ppi | Screen-only viewing, smallest files |
Size limits you’ll run into
- Email: Gmail and Yahoo accept attachments up to 25 MB, Outlook.com 20 MB, many company mail servers only 10 MB.
- Job and university portals (Workday, Greenhouse, university applications…) often limit uploads to 2–5 MB per file.
- Government and visa forms frequently require PDFs under 1 MB or even 500 KB — use “Compress to a target size”.
- Scanned documents shrink the most: try the Balanced level with “Convert images to grayscale”.
- Keep the original of important documents: compression is one-way. Your file on disk is never changed by this tool.
