Optimize · 5 min
How to compress a PDF without losing quality
Why PDFs get heavy, what compression actually does, and how to shrink files for email and the web.
What makes a PDF heavy?
Most large PDFs are heavy for one reason: embedded images stored at print resolution. A 300 DPI scan of an A4 page is roughly 8 MP — multiply that by 50 pages and you're looking at hundreds of megabytes before a single line of text.
How browser-based compression works
The Compress PDF tool rebuilds your PDF by:
- Re-encoding embedded JPEGs at a lower quality.
- Down-sampling oversized images to a screen-sensible DPI.
- Stripping redundant metadata and bookmarks duplicates.
- Rewriting the cross-reference table with object streams.
The result is usually 40–70% smaller with no visible difference at normal reading zoom.
When compression doesn't help
If your PDF is mostly vector text (think academic papers, generated reports), there isn't much to compress. In that case, look at:
- Removing unused pages via Remove Pages.
- Embedding only the font subsets you actually use.