Compress Scanned PDF

A scanner set to its default "high quality" preset commonly captures at 300 DPI or higher, which is calibrated for print reproduction — the resolution needed so that fine text and detail survive being physically printed again. A document that will only ever be viewed on a screen has no use for that resolution, because no monitor displays anywhere near 300 dots per inch at normal viewing distance; the extra pixel data is simply discarded by the display without ever being seen; 120 DPI is comfortably above what a screen can resolve for reading purposes while carrying a fraction of the pixel count, which is where the bulk of the size reduction comes from.

The quality setting on top of the DPI reduction addresses the JPEG-style compression applied to each page image afterwards — 70 is a level where the occasional faint blocking around dense text is not distracting on screen, while still compounding with the resolution drop for a combined saving in the 70–90% range on a typical multi-page scan.

If a scanned document will ever need to be printed again, or if it contains fine print, small diagrams or handwritten annotations that need to stay legible when zoomed in, a higher DPI — 150 to 200 — preserves that legibility at a proportionally smaller saving than this preset's aggressive default. The trade is always the same one: resolution kept versus file size given up, and a document destined only for a screen and an email attachment rarely needs to sit anywhere near the print-resolution end of that trade.