How to compress a PDF for email: what actually shrinks it
By Altaf Ahmed. Written on . Posted in Tutorials.
Gmail refuses attachments over 25 MB and Outlook.com stops at 20 MB. On top of that, email encodes attachments in Base64, which adds roughly a third to their size in transit, so a PDF that is comfortably under the limit on disk can still bounce. Some company mail servers set lower limits than either.
To find out what compression actually does, we ran three typical documents through the same Ghostscript settings our Compress PDF tool uses.
The test
- Photo brochure: 8 A4 pages, each a full-page photo embedded at 300 dpi. 8.7 MB.
- Scanned document: 6 pages of text scanned in colour at 300 dpi, the way most office scanners and scanning apps save them. 18.3 MB.
- Text-only PDF: 20 pages exported straight from a word processor, no images. 14 KB.
The tool's three levels map to Ghostscript's standard presets: Low keeps images at 300 dpi, Recommended downsamples them to 150 dpi, and High to 72 dpi.
Results
| Document | Original | Low | Recommended | High |
|---|---|---|---|---|
| Photo brochure | 8.7 MB | 4.3 MB (−50%) | 375 KB (−96%) | 123 KB (−99%) |
| Scanned document | 18.3 MB | 18.2 MB (−0.1%) | 1.3 MB (−93%) | 468 KB (−97%) |
| Text-only PDF | 14 KB | 22 KB (+56%) | 14 KB (−1%) | 14 KB (+1%) |
What the numbers tell you
1. Images are almost the entire file size
The scanned document is six pages of plain text, yet it weighed 18.3 MB, because each page is stored as a full-colour photograph of the paper, texture and scanner noise included. Reduce the resolution of those page images and the file collapses. The text-only PDF, with no images, had nothing to remove.
2. "Recommended" is the right default
At 150 dpi the scanned text was still clean and fully readable at normal zoom. It took the 18.3 MB scan to 1.3 MB, small enough for any mail service.
3. "High" is for when size matters more than detail
At 72 dpi the same scan dropped to 468 KB, but the letters became visibly rough. That's acceptable for a receipt or a form you just need someone to read, but don't use it for small print, fine diagrams, or anything that will be printed.
4. Compressing can make a file bigger
In our test, rewriting the text-only PDF at the Low setting made it 56% bigger, because rewriting a PDF that is already efficient adds overhead rather than removing any. Our tool checks for this: if the result isn't smaller, it tells you your original is already as small as that setting can make it, instead of handing you a bigger file.
Step by step: get a PDF under the limit
- Open Compress PDF and choose Recommended.
- Check the result. If it's still too big and the document is a scan or mostly photos, try High and zoom in to check the text is readable.
- If a colour scan only contains black text, converting it with Grayscale PDF can shave off more.
- Still too large? Remove pages the recipient doesn't need, or split the PDF and send it in parts.
What happens to your file
Compression runs on our server, because it relies on Ghostscript. Files up to 50 MB are accepted, processed, and deleted immediately after the compressed version is sent back to you. Nothing is stored.