How to Shrink a Scanned PDF Without Losing Legibility
Scanned documents are usually the biggest offenders when it comes to bloated PDF file sizes — a 10-page scan can easily outweigh a 200-page text document. Here's how to shrink one down without turning the text into a blurry mess.
Why scanned PDFs are so much bigger than "normal" ones
A text-based PDF stores letters as instructions — draw this character, at this position, in this font. That's tiny. A scanned PDF, on the other hand, is really just a series of full-page photographs stitched together. Every page is a high-resolution image, often saved at settings meant for print quality rather than screen viewing, which is why file sizes balloon so quickly.
This also explains why compression affects scanned PDFs so differently than typed ones: there's a lot of image data to work with, so the potential savings are much larger — but push too hard and text can turn fuzzy or hard to read.
The balance: strong compression, still legible
The goal with a scanned document isn't the smallest possible file — it's the smallest file that's still comfortably readable. Scanned text, in particular, tends to hold up fine at moderate compression but degrades noticeably once you push into the most aggressive settings, since fine details like serifs and small print are the first thing to blur.
Step-by-step guide
- Open the Compress tool and drop in your scanned PDF.
- Click "Estimate output sizes" to preview roughly how large the file will be at each setting.
- Start with "Recommended" for scanned text — it's the setting most likely to balance size and legibility well.
- Download and check — zoom in on a paragraph of small text to make sure it's still comfortably readable.
- If it's still too large, or you have a hard size limit to hit, enter your target size in MB and click "Pick closest" instead.
When compression alone isn't enough
If a scanned document is still too large even at the strongest setting, it's often because it was scanned at a much higher resolution than necessary (some scanners default to 600 DPI or higher, when 200–300 DPI is plenty for a readable document). Unfortunately, resolution reduction has to happen at scan time — a compression tool can't add back detail that a lower-resolution rescan would have captured more efficiently. If you have the option to rescan at a lower resolution, that combined with compression afterward will usually get you the smallest usable file.
Frequently asked questions
Which preset should I use for scanned documents specifically?
"Recommended" is a good starting point for most scanned text. Use "Low" if legibility matters more than file size (e.g. a contract with fine print), and reserve "High" for cases where hitting a strict size limit matters more than perfect clarity.
Why does my scanned PDF barely shrink at all?
This is unusual for scans, and usually means the file was already compressed by your scanning app before you downloaded it. There's a floor to how much any image can be compressed before quality visibly suffers, so a file that's already near that floor won't shrink much further.
Is compressing a scanned legal or financial document safe?
Yes — PDFLantern's tools run entirely in your browser using JavaScript. Your file is never uploaded to a server or seen by anyone else; it's processed locally on your own device.
Will compression make signatures or stamps on a scanned document hard to verify?
Moderate compression (Low or Recommended) typically preserves signatures and stamps well enough to remain visually clear. If a document's authenticity depends on fine visual detail, preview the result at full zoom before relying on the compressed copy.