Most of a PDF's weight is images. Compression works by lowering their resolution, so the only real question is how much detail you can afford to lose.
The levels
- Low (300 DPI) — safe for printing. Modest savings.
- Medium (150 DPI) — the default. Good for email and archiving.
- High (72 DPI) — screen only. Photos will visibly soften.
- Custom — set the resolution yourself.
When compression does nothing
A PDF that is mostly text is already small, and squeezing it further gains almost nothing. If our compressed result would be larger than your upload, we hand back the original instead.