If you've ever scanned a few pages and ended up with a 30 MB PDF, you're not alone. Scanned PDFs are notorious for being huge — often ten or twenty times larger than the same document typed out. Understanding why explains exactly how to fix it.
A scan is a photograph, not text
When you scan a page, the scanner doesn't "read" the words. It takes a high-resolution photograph of the page and embeds that image in the PDF. So a scanned contract isn't text and lines — it's a stack of large pictures. Every page is a full-color (or grayscale) image, usually captured at 300 DPI or higher.
At 300 DPI, a single A4 page is roughly 2,480 × 3,508 pixels — about 8.7 megapixels per page. Multiply that across 20 pages and it's no surprise the file is enormous.
The two levers that control scan size
- Resolution (DPI). Higher DPI = more pixels = bigger file. 600 DPI is four times the data of 300 DPI. For documents meant to be read on a screen, 150 DPI is usually plenty.
- Color depth. A full-color scan of a black-and-white document wastes enormous space. Many scans can be reduced to grayscale with no real loss.
How to shrink a scanned PDF
The fix is to downsample the page images to a sensible resolution and recompress them efficiently. Dropping a 300–600 DPI scan to around 150 DPI typically cuts the file by 70–90% while staying perfectly readable on any screen and fine for most printing.
With slimdocs, just drop the scanned PDF into the compressor and choose a level. Because scanned PDFs are pure images, they respond extremely well to compression — this is the case where you'll see the most dramatic savings, often 80% or more at the Maximum or Extreme levels.
What about editing the text?
A normal scan has no real text layer — it's just pictures — so you can't select or search the words. If you need an editable, searchable result, you need OCR (optical character recognition), which detects the letters in the image and adds a real text layer. slimdocs applies OCR automatically when you convert a scanned PDF to Word, so the output contains genuine, editable text rather than a picture of the page. See our guide on converting PDF to Word.
Quick checklist
- Scan at no more than 300 DPI if you can — and grayscale for text documents.
- Run the result through the slimdocs compressor.
- Choose Balanced or Maximum; check it's still readable.
- If you need editable text, use PDF → Word, which adds OCR.