You try to email a 28 MB PDF and the attachment bounces. Compression should fix it, but results vary wildly: one file shrinks to 2 MB, another barely moves. The difference comes down to what is actually inside the file.
What makes a PDF large
In almost every oversized PDF, images are the culprit. A phone photo dropped into a document can be 4000 pixels wide, even though it is displayed at the size of a postage stamp. Text, by contrast, is tiny: a hundred pages of plain text is often under 1 MB.
- Scanned documents: every page is a full-resolution image, so these compress very well.
- Slide decks and brochures: many large photos, also very compressible.
- Text-only reports exported from Word: already small, so expect only modest savings.
Choosing a level
Compress PDF offers several levels. Lighter levels clean up the file structure and keep images untouched. Stronger levels re-encode and downsample images, trading some sharpness for much smaller files.
- Email and messaging: a medium level is usually enough to get under common 10–25 MB limits.
- Archiving scans you will read on screen: strong compression is fine.
- Print: use the lightest level, because downsampled images can look soft on paper.
Privacy while compressing
Large files are often the sensitive ones: scanned contracts, medical records, tax returns. Compressing in the browser means those pages are never uploaded to a third-party server just to make them smaller.