Why your PDF looks blurry after compression — and how to fix it
Compression is a trade, not magic. Something was discarded to make the file smaller, and if the result looks wrong, it is worth knowing exactly what went and whether it can come back.
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What a compressor actually changes
Real text in a PDF is vector data — outlines and coordinates. It is tiny already and compressors leave it alone, which is why a text document barely shrinks and never blurs.
Images are a different story. Compressors downsample them to a lower resolution and re-encode them as JPEG at a reduced quality. Both steps discard pixel data permanently.
Why scans suffer most
- A scanned page is one big image, so the text on it is pixels, not outlines — and pixels are exactly what compression attacks.
- JPEG handles smooth gradients well and sharp black-on-white edges badly, which is why compressed scans get grey haloes around letters.
- Downsampling from 300 DPI to 96 DPI removes two thirds of the detail in each direction. Small print goes first.
- Repeated compression compounds: each pass re-encodes the already-degraded result.
How to get a readable file back
Start again from the original. Blur cannot be undone, so the fix is always to re-compress the source at a gentler setting rather than to sharpen the damaged output.
If the medium setting is still too aggressive, stop compressing and change strategy: split the document and send the relevant pages at full quality, or re-scan at a moderate resolution rather than scanning huge and shrinking hard.
Step by step
- 1Find the original, uncompressed PDF — the compressed copy cannot be repaired.
- 2Re-run the compressor below one level lighter than before.
- 3Zoom to 100% and check the smallest text on the busiest page.
- 4Repeat one level at a time until the size target and legibility meet.
- 5If they never meet, split the document instead and send only the pages that are needed.
Frequently asked questions
- Can I un-blur a compressed PDF?
- No. The discarded pixel data is gone. Sharpening filters increase contrast at edges but cannot restore detail, and usually make scans look worse.
- Why did my text-only PDF barely shrink?
- Because there was nothing image-heavy to compress. Text is already extremely compact; a 200 KB text document is close to its floor.
- What scan resolution should I use to avoid this?
- Scan documents at 200-300 DPI. Higher makes huge files that then need aggressive compression; lower makes small print marginal from the start.