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Most compression tools default to “lossy” without telling you. That’s fine for the average user, but it can ruin a deliverable when the file is going to a designer, an engineer, or a printer.
Lossless: same pixels, smaller container
Lossless compression reorganises the PDF without touching image data. It strips unused fonts, deduplicates repeated images, removes orphaned objects, and re-encodes streams with stronger Flate compression. Output is byte-different but pixel-identical.
Expect 10–35% size reductions. The exact number depends on how much waste was in the original.
Lossy: rewriting images for the web
Lossy compression downsamples images to a target DPI (typically 150 for screen, 300 for print) and re-encodes them with stronger JPEG / JBIG2 quantisation. The resulting images are visually almost-identical at normal viewing distance but mathematically different.
Expect 50–80% reductions on image-heavy documents.
The decision tree
- Going to a designer, an engineer, or a print shop? Lossless.
- Archiving the master copy? Lossless.
- Sending by email or uploading to a portal? Lossy with sensible defaults.
- Mostly text with a logo here and there? Either works — start lossless and only escalate if the result is still too big.
What to test
Open the compressed file at 100% zoom and at 200% zoom. If text edges and image gradients still look the same, you’re fine. If you see banding or fuzziness only at 200%, that’s normal — your audience won’t be looking that closely. If you see it at 100%, the preset was too aggressive.
Frequently asked questions
Can I recover a lossy-compressed PDF?
No. Lossy is one-way. Always keep an unmodified original if you might need higher quality later.
Why does my "lossless" output look softer?
It shouldn't. If it does, the tool isn't actually running lossless — it's applying a quality cap. Check the preset.