An oversized PDF bounces from email, crawls when uploaded, and eats storage. The good news: most bloat comes from a few predictable places, and trimming it rarely means visibly worse quality. This guide explains what actually makes a PDF large and the highest-impact ways to shrink it.
What makes a PDF large
- High-resolution images: A phone photo at 4000×3000 px embedded at full size dwarfs the text around it. Images are usually 80–95% of a bloated file.
- Embedded fonts not subsetted: Full font files carry thousands of glyphs you never use.
- Scanned pages: Each page is a full-resolution image.
- Redundant data: Old revisions, unused objects, and metadata linger after edits.
The highest-impact fixes
- Downsample images. For screen and normal printing, 150–200 dpi is plenty. Dropping a 600 dpi image to 150 dpi can cut its size by 90% with no visible change on screen.
- Subset fonts. Embed only the glyphs actually used. Most export tools do this automatically if you enable it.
- Compress images with JPEG/JPEG2000 for photos; keep line art and text screenshots in a lossless format to avoid fuzz.
- Remove unused objects and metadata using an "optimize" pass.
Tools that do it
- Adobe Acrobat Pro: File → Save As Other → Reduced Size PDF, or the more granular PDF Optimizer.
- macOS Preview: File → Export → Quartz Filter → "Reduce File Size" (be aware its default can over-compress images).
- Free/offline: Ghostscript with a downsampling preset, or LibreOffice's PDF export with reduced image resolution.
Target sizes for email
| Use | Target size |
|---|---|
| Email attachment (safe) | Under 10 MB |
| Email attachment (upper limit) | 20–25 MB (often blocked above) |
| Web upload / forms | Under 5 MB where possible |
| Anything larger | Share a link instead of attaching |
Smaller files also process faster in unlock and other tools, a relevant bonus if you're near a tool's size limit.