Scanned PDFs are the heavyweights of the document world. A handful of scanned pages can balloon to tens of megabytes, because each page is stored as a full-resolution image rather than as text. The good news is that scans are also the files that compress the most dramatically. This guide explains how to shrink a scanned PDF and what to watch out for.
Why scans are so large
When you scan a document, your scanner captures each page as a photograph, often at 300 or 600 DPI. There is no text in the file, just pixels, and lots of them. A single color page at 600 DPI can be several megabytes on its own. Multiply that across a long document and the file size explodes.
Because the bulk is image data, scans respond extremely well to image compression. This is the opposite of a text-based PDF, which has little to shrink. For the contrast, see why is my PDF so large.
Compressing a scanned PDF
Open the Compress PDF tool and drop in your scan. You have two paths:
- Standard mode recompresses and downsamples the embedded scan images while keeping the file structure intact. Choose Recommended or Strong depending on how small you need it. For many scans this alone cuts the size by half or more.
- Maximum compression, the toggle on the tool, rasterizes each page into a single optimized image. For scans this is often the most effective option, because the page was already an image anyway, so you lose nothing extra by flattening it.
For a typical scanned report, try Strong first. If you need it even smaller, enable Maximum compression and compress again.
DPI is the dial that matters
The biggest factor in a scan's size is resolution. A document scanned at 600 DPI looks identical to one at 150 DPI on a normal screen, but takes roughly sixteen times the space. Compression effectively downsamples those oversized pages to a sensible resolution. That is why a giant scan can shrink so much while still being perfectly readable.
If you plan to print the scan, keep the resolution higher and use Light or Recommended. If it is for screen reading or email, Strong is usually fine.
Make scanned text searchable
Compression makes a scan smaller, but it does not make the text inside it selectable, because there is no text, only pixels. To get searchable, copyable text out of a scan, you need OCR (optical character recognition), which is a separate step. Learn more in AI vs traditional OCR. A common workflow is to OCR first, then compress.
Watch out for over-compression
Scans of text can become hard to read if you compress too aggressively, because letters are made of fine detail that lossy compression blurs. If the Strong or Maximum result looks muddy, step back to Recommended. As with any lossy process, always keep your original scan in case you need the full-quality version later. See lossy vs lossless PDF compression for why this matters.
Privacy for sensitive scans
Scanned documents are often the most sensitive ones you handle: passports, medical forms, signed contracts. Because the Compress PDF tool works entirely in your browser, those scans never get uploaded to a server. For more on that, read private in-browser PDF compression.
Summary
Scans compress beautifully because they are pure image data. Use Strong or Maximum compression in the Compress PDF tool, keep an eye on readability, OCR first if you need searchable text, and always retain the original.
