Where the size goes
Most PDF bloat comes from high-resolution images and embedded fonts. Compression mainly attacks the images — downscaling and re-encoding them — which is why a photo-heavy scan shrinks far more than a text document.
Resolution vs quality
The Compress tool gives you two dials: target resolution (DPI) and JPEG quality. Lower resolution removes pixels; lower quality removes detail within them. Start at 110 DPI and 70% quality, then adjust.
Reading the result
After running, the tool reports the before and after size and the percentage saved. If text looks soft, nudge resolution up; if the file is still huge, bring it down.
When not to compress
If your PDF is mostly vector text, compression has little to work with and may not help. It shines on scanned documents and image-heavy reports.
