How to Reduce the Size of a Scanned PDF

Scan a ten-page contract and you’ll often get a 25 MB file — too big to email, too big for most upload forms. Scans are the single worst case for PDF size, but also the case where compression works best.

Why scans are so heavy

A scanned PDF contains no text at all. Every page is a bitmap photograph, usually captured at 300–600 DPI in full colour — even when the page is black ink on white paper. That’s megabytes per page for content that, as real text, would be a few kilobytes.

That’s also why the usual “lossless” compression barely dents a scan: there’s no redundant structure to tidy up. The image itself has to be re-encoded.

The fix: re-encode the pages

Open Compress PDF, pick your scan, and choose Strong mode. It renders each page and re-encodes it as an optimized JPEG — typically cutting scanned documents by 70–90% with no visible difference at reading size. A 25 MB scan commonly lands around 2–4 MB.

Three things worth knowing before you click:

1. The text in a scan was never selectable — it’s a photo. So unlike with born-digital PDFs, Strong mode costs you nothing: there’s no live text to lose.
2. Drop the junk pages first. Scanners love adding blank backsides and separator pages. Deleting them before compressing multiplies your savings.
3. Scan smarter next time. If you control the scanner, 150–200 DPI greyscale is plenty for documents and produces files a fraction of the size.

Keep sensitive scans off other people’s servers

Think about what you scan: contracts, IDs, medical letters, tax forms. Upload-based compressors send all of that to their servers before giving you a smaller file back. The compressor here does the re-encoding entirely in your browser — your scan never leaves your device, which you can verify in the Network tab while it runs.

Compressed it to email it? Gmail and Outlook cap attachments around 25 MB — the compress-for-email guide covers the details.