What PDF compression actually does
Most of a PDF's weight is images. Compression downsamples them to a lower resolution, re-encodes them as JPEG, and drops duplicate objects, unused metadata, and embedded font data that is never rendered.
Text and vector lines are stored as instructions, not pixels, so they take very little space to begin with. That is why a 40-page text-only document can be smaller than a single scanned page.
When compression helps and when it does not
The largest savings come from files built out of scans or photos.
- Helps: scanned contracts, photo-heavy reports, slide decks exported with uncompressed images, files with the same logo repeated on every page.
- Barely helps: text-only PDFs already under about 1 MB, files already run through another compressor, documents with a single small image.
Encrypted PDFs must be unlocked before they can be compressed.
Choosing a compression level
Screen levels shrink the most and are fine for email or web upload; small print in scanned pages may become fuzzy. Print levels keep higher resolution and produce a larger file.
No setting guarantees a specific file size. Test the level on one file, check it, then apply the same level to the rest.
What to check after compressing
Open the compressed file and compare it with the original before you send it.
- Page count and page order
- Small print, footnotes, and table borders
- Form fields, links, and bookmarks
- Digital signatures, which usually break when the file is rewritten