Your PDF is 60 MB and your email won't send it. You hit 'compress', wait a few seconds, and now it's 5 MB. What just happened? Here's the friendly explanation.
What compression does
PDF compression mostly works on images — downsampling high-resolution scans and photos to screen-friendly sizes. A 600 DPI scan becomes 200 DPI, visually identical but dramatically smaller. It also re-encodes images more efficiently and strips dead weight (old metadata, unused fonts, duplicate resources). Text usually doesn't compress much because it's already small; image-heavy PDFs see the biggest gains.
Choosing a quality level
Most tools offer presets: 'screen' (smallest, fine for email and web), 'ebook' (medium, readable everywhere), 'print' (largest, print-ready). For email attachments and casual sharing, screen quality is invisible to the eye and friendly to your inbox. For archival or print-bound files, keep print quality. Always work on a copy — compression is one-way, and you might need the high-res original later.
What to expect
Text-only PDFs typically shrink 10-30%. Mixed text and images: 40-70%. Heavy scans: 80-95%. If your file barely shrinks, it's already near its floor. If it shrinks dramatically, the source was bloated (high-res images, redundant data). Compression is non-destructive in the sense that the document remains usable; it's lossy for images, so visual fidelity drops slightly at aggressive settings.
FAQ
Will my recipient notice?
At default settings, no — text stays crisp, photos slightly softer but acceptable. At aggressive settings, recipients might spot pixellation in images. For professional sends, test on a sample page first.
Can I compress a signed PDF?
Compressing usually invalidates digital signatures because the file changes. Compress before signing, not after. For electronic (drawn) signatures, compress is usually fine but check the visible signature still reads cleanly.
What if compression doesn't shrink enough?
Either the file is already optimised, or you need to split into parts. For huge scans, also reduce resolution at scan time — 200 DPI is fine for text, no need for 600.
Compression turns email-rejected PDFs into email-friendly ones. Try Flint on your biggest file.