PDF compression explained: how to shrink a PDF without ruining it
Understand DPI, JPEG quality and what really makes a PDF large, so you can pick the right compression level for email or upload limits.
Quick answer
PDFs are large mostly because of images. Compress PDF rebuilds each page as an optimized image at the resolution you pick: 72 DPI for the smallest files, 110 DPI for a balance, 150 DPI for print-friendly quality. Text becomes part of the image, so keep the original if you need searchable text.
What makes a PDF big?
- Scanned pages and photos — by far the biggest factor. A single 300 DPI colour scan can be several MB.
- Embedded fonts — usually tens or hundreds of KB.
- Text and vector graphics — tiny. A 100-page text-only PDF may be under 1 MB.
Choosing a level
| Level | DPI / quality | Good for |
|---|---|---|
| Strong | 72 DPI, quality 0.5 | Email attachments, upload portals with tight limits, reading on phones |
| Balanced | 110 DPI, quality 0.65 | General sharing; text remains comfortably legible |
| Light | 150 DPI, quality 0.8 | Printing at home, documents with fine print |
The trade-off
Our compressor uses a rasterising method: each page is drawn as an image and re-encoded. That's what makes 50–90% reductions possible on scans. The cost is that text is no longer selectable or searchable, and links, form fields and bookmarks are dropped. The tool asks you to confirm this before compressing.
If the compressed file ends up larger — common for text-only PDFs — the tool tells you and you can simply keep the original.