You hit 'compress', wait a few seconds, and a 60 MB PDF becomes 4 MB. Same pages, same content, dramatically smaller file. Magic? Not quite — here's what the tool is actually doing.
Image downsampling
The biggest win is almost always reducing image resolution. A 600 DPI scan becomes a 200 DPI version — visually identical on screen, dramatically smaller in bytes. Compress PDF lets you choose: 'screen' (small, web-fine), 'ebook' (medium, readable everywhere), 'print' (larger, print-ready). For email attachments and casual sharing, screen quality is invisible to the eye and friendly to your inbox.
Image re-compression
Beyond resolution, compression tools re-encode images more efficiently. JPEG quality drops slightly, PNG compression goes harder, redundant copies of the same image get deduplicated. None of this affects text — only embedded pictures. That's why text-heavy PDFs see modest gains while photo-heavy ones can shrink 90%.
Cleanup: fonts, metadata, leftovers
PDFs collect cruft as they're edited — old form data, embedded fonts you removed, thumbnails, previous revisions, metadata. Compression tools strip the dead weight. This bit is lossless: nothing visible changes, the file just stops carrying ghosts. Combined with image work, this is why fresh re-saves through Flint often run lighter than the original.
FAQ
Why won't my PDF compress further?
Either the images are already low-resolution, or the file is pure text (which doesn't compress much). Heavy text PDFs are usually already near their floor. Adding more compression won't shrink them and risks artefacts.
Does compression hurt quality?
It can, if pushed too hard. Default settings target invisible-to-the-eye loss. Aggressive settings start to soften text and pixellate photos. Match the preset to the purpose: email = screen quality, print = print quality.
Can I undo compression?
Not really — compression is a one-way trip. Always keep the original if you might need the high-resolution version later. Run compression on a copy, not the master.
Compression mostly means smarter images plus a quick tidy-up. Try it in Flint — most files shed half their weight in seconds.