Compress a scanned PDF properly

Every “compress PDF” marketing page implies the same promise: any PDF in, magic smaller PDF out. The truth has a clear line down the middle — and which side your file sits on decides everything.

Two kinds of PDF

Scanned PDFs are photographs bound into PDF form: each page is one big image with no text layer. Because the underlying JPEGs were saved by the scanner at conservative quality, re-encoding them at searched quality+scale shrinks files dramatically — 5 MB scans regularly land at 300–800 KB with no readable loss. This is the case rasterising compressors are made for.

Digital PDFs are fonts, vectors and programmatic text. Re-encoding them means rendering every page to an image and losing exactly the properties PDFs exist to keep: selectable text, small file size, crisp zoom. A 100 KB text PDF re-rendered as JPEGs at readable quality typically comes out larger than it went in.

The honest decision tree

1. Is there a text layer? A good tool probes this — ours (compress PDF) checks and states it right on the result.
2. Scanned: compress towards the target. The engine searches quality/scale per page, stops at the best under-target attempt, and notes “Scanned document detected” on the card.
3. Digital: you have three honest options — keep it (it may be tiny already; the tool reports “already optimised / no gain” rather than bloating it), rasterise deliberately when you accept losing text-selectability and infinite zoom, or re-structure losslessly with split / merge to send only the pages a recipient needs.

What you can expect in KB

• Black-and-white letters/documents scanned at 300 ppi: 40–150 KB per page after honest compression.
• Colour scans with photos or signatures: 100–400 KB per page.
• Total file: pages × per-page, so a ten-page B&W scan commonly fits under 1 MB — comfortably below most mail and portal caps.

The verification habit

Before sending a compressed scan, open it and try to read the smallest text present. Then — since compression happens on your device with our suite — check that text pages you did not mean to rasterise kept their selectable text (the result card will tell you, but the 5-second check is free). For capped uploads where the cap keeps refusing, split the document: portals rarely need all 40 pages at once, and page ranges take one line to enter.