How to compress a PDF

A PDF is usually large for one reason: images. Text and vectors are tiny; a few high-resolution photos or a scan can balloon a file to many megabytes. Effective compression targets the real culprit.

What makes PDFs big

• High-resolution images — the number one cause.
• Scans saved as full-colour images at high DPI.
• Fully embedded fonts that aren't subsetted.
• Leftover data from incremental edits and unused objects.

Ways to shrink a PDF

• Downsample images to a sensible resolution — 150 DPI is plenty for screen reading; 300 DPI for print. This gives the biggest wins.
• Recompress images (e.g. JPEG for photos) at a reasonable quality.
• Subset fonts so only used glyphs are embedded.
• Clean and re-save the file to drop unused objects and old revisions — see flattening.

Quality vs size

Compression is a trade-off. Aggressive downsampling makes text in scans fuzzy and images blocky. Match the resolution to the destination: don't ship a 600-DPI scan just to be read on a phone. Test the result at 100% zoom before sending.