What Makes PDFs Large?
PDF size is dominated by embedded images. A scanned document at 300 DPI stores each page as a high-resolution image — a 10-page scan can easily exceed 20 MB. Other contributors:
- Embedded fonts (typically 50–500 KB each)
- Uncompressed or losslessly compressed page streams
- Redundant resources (fonts or images referenced but unused)
- Metadata and version history from PDF editors
Image Re-encoding: The Main Lever
Most PDF compression tools work by extracting embedded images and re-encoding them as JPEGs at a lower quality setting (typically 60–80%). This is lossy — there is a quality tradeoff — but at 75% JPEG quality, the reduction in visible quality is minimal while file size can drop 60–80%.
Compression Levels
| Level | Quality | Typical size reduction | Best for |
|---|---|---|---|
| Low compression | High | 10–30% | Archiving, printing |
| Medium compression | Good | 40–60% | Email attachments |
| High compression | Acceptable | 60–80% | Web upload size limits |
When Compression Doesn't Help
Compression has minimal effect on PDFs that are already image-efficient: text-only documents, vector graphics, or PDFs exported from a word processor. These are already well-compressed. Compressing a 50 KB invoice PDF may yield a 48 KB result — not worth the quality tradeoff.
Compress a PDF Now
Open ToolsVito's Compress PDF, choose a compression level, and download the optimized file — re-encoded locally, nothing uploaded.