Compression (PDF)
Techniques for reducing PDF file size — by re-encoding images at lower quality, removing unused objects, or both.
PDF compression means reducing the size of a PDF file — typically because it's too large to email or slow to upload. There are several categories of compression, each with different trade-offs.
Types of compression
- Image re-encoding. The biggest wins come from recompressing embedded images. A photo saved at 300 DPI can often be downscaled to 150 DPI with no visible difference on screen. Image-heavy PDFs can shrink 70-90% with aggressive compression.
- Stream compression. PDF internally uses zlib-like streams. Already compressed by default on most PDFs.
- Object deduplication. Removing unused fonts, duplicate images, and embedded metadata that isn't needed.
- Font subsetting. Embedding only the characters actually used from each font, not the entire font file.
Trade-offs
Compression always trades file size for quality. Our Compress PDF tool offers three levels — Less (20-40% reduction, minimal quality loss), Recommended (50-70% reduction, slight quality loss), Extreme (70-90% reduction, visible quality loss on images).
What compression cannot do
If your PDF is 10 MB of pure text, no compression tool will get it under 1 MB — there's nothing left to compress. Compression gains come from image data; a text-only document is already efficient.
Tools
- Compress PDF — three quality levels, runs in your browser
- Resize PDF — change page dimensions (different from compression)