Why a scanned PDF is so much bigger than it looks
A scanned document isn’t really “text in a file” the way a Word export or a typed-and-saved PDF is. Every page is a photograph — a full-resolution image of the paper, stored inside the PDF one page at a time. A five-page scan at a typical scanner’s default settings can easily run 10–20 MB, even though the same five pages of typed text would be a few hundred kilobytes.
That’s the core problem with “just compress it”: there’s no hidden slack to remove from text that isn’t there. The only real lever is the image itself — its resolution and how aggressively it’s re-encoded.
What actually shrinks a scanned PDF
Each compression attempt does two things at once: it repacks the file (dropping unused objects and recompressing its internal data) and re-encodes the page images as JPEG at a chosen quality. In a scan, almost all of the bytes are those images, so the JPEG quality is what decides the final size. That’s a trade-off: a very small target on a many-page scan means visible compression artifacts.
Working backward from a target size means the tool tries several quality levels and keeps the highest one that still fits, instead of applying one fixed “medium” setting to every file. If even its lowest allowed quality doesn’t fit, it redraws each page as a compressed image as a last resort, at 72 dpi and, if that’s still too big, at half that. If your scan has a searchable text layer from OCR, that text stops being selectable when this happens, and the result screen tells you.
Getting a specific size out of a scan
If you have an exact number to hit — a portal that says “under 200 KB,” for instance — set that as your target rather than guessing at a quality percentage. A single-page scan usually has plenty of room to shrink well below a small target; a long multi-page scan has much less room per page, since every page’s bytes count toward the same total.
The tool won’t go below a set minimum quality. If a scan still doesn’t fit at that point, it tells you and hands back the smallest version it made, labeled as a best effort. Open it and check the text is still readable before you submit it.
Try it
Compress a scanned PDF now. Pick your target size, and you’ll see the measured result before you download anything. Sending it by email? See realistic email attachment limits before you pick a target.