A PDF that won't attach to an email or keeps failing an upload limit is almost always too big because of its images — scanned pages and high-resolution photos, not the text. The good news is you can usually cut the file size by half or more without any visible loss.
This guide explains what actually makes a PDF large, how to compress one safely, and how to avoid the blurry, over-compressed result that makes a document look cheap.
What you need
- ✓The PDF you want to shrink.
- ✓A PDF compressor (a browser-based one keeps the file private — nothing is uploaded).
- ✓A rough target size — e.g. under 10 MB for most email, under 5 MB for many web forms.
Step-by-step
- 1
Check what's making it big
Text-only PDFs are already small. If yours is large, it's almost certainly scanned pages or embedded photos. That tells you compression (which targets images) will help a lot.
- 2
Open the PDF in a compressor
Use a browser-based PDF compressor so the file is processed on your device and never uploaded — important for anything private like contracts or bank statements.
- 3
Pick a compression level
Start with 'medium' or around 150 DPI. This re-samples images down to screen resolution, which is invisible on a monitor but dramatically smaller than print resolution.
- 4
Compress and check the result
Run it, then open the output and zoom to 100%. Text should stay crisp; photos should look clean. If it's fine, you're done.
- 5
Only go further if you must
If it's still too big, drop to a higher-compression setting, but review carefully — this is where images start to look soft. Better to split the PDF than to over-compress.
Examples
- A 28 MB scanned contract compressed at medium settings drops to about 4 MB — small enough to email, with text still perfectly legible.
- A 12 MB brochure full of photos compressed to ~3 MB for a web upload, with no visible difference on screen.
Tips
- →Compress a copy, never your only original — compression is lossy and can't be undone.
- →For documents you'll only ever read on a screen, 150 DPI is plenty; save 300 DPI for files you'll print.
- →If one huge image is the culprit, replacing it with a smaller version before making the PDF beats compressing the whole file.
- →Splitting a long PDF into parts is often better than crushing the quality to hit a size limit.
- →Black-and-white scans compress far smaller than colour — scan in grayscale if colour isn't needed.
Common mistakes
- Compressing the only copy. Always keep the original; work on a duplicate since compression is irreversible.
- Using the highest compression by default. Start at medium — maximum compression is where text and images visibly degrade.
- Uploading a private PDF to an unknown site. Use a browser-based tool that processes the file locally so it's never uploaded.
- Compressing an already-small text PDF. If it's already a few hundred KB, there's little to gain — the size is the text, not images.
Conclusion
Compressing a PDF is mostly about re-sizing its images to the resolution you actually need. Work on a copy, start at medium, and check the result at 100% — you'll usually hit your size limit with no visible loss, and if not, splitting the file beats crushing the quality.