Compress a Large Scanned PDF Without Losing Legibility

Scanned documents are almost always the largest PDFs you'll deal with, for a simple reason: every page is stored as a full-resolution image rather than lightweight text. A 50-page scanned contract can easily run into hundreds of megabytes, where the same document as native digital text would be a fraction of that size.

This is also exactly the case where compression has the biggest impact, since re-rendering each page at a lower resolution directly attacks the thing making the file huge. The four presets trade detail for size differently: Low (300 DPI, 0.92 quality) barely shrinks a scan but keeps it near-original; Medium (200 DPI, 0.82 quality) is the usual starting point for a large scan; High (150 DPI, 0.70 quality) gets aggressive; Maximum (96 DPI, 0.50 quality) produces the smallest file but can make small text genuinely hard to read.

That last point matters more for scans than for anything else: a scanned page has no underlying selectable text to fall back on if the image gets too degraded — what you see in the compressed output is all there is. For a scan with small print or fine detail (a receipt, a dense legal document), start at Medium and only go to High if you genuinely need the smaller file, checking legibility at actual size before committing to a setting.