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How to compress a scanned PDF

Scanned PDFs are usually mostly image — and image PDFs shrink dramatically. Here's how to compress a scanned PDF in your browser without sacrificing legibility.

Open the Compress PDF tool →

Scanned PDFs are the biggest size offenders in the PDF world. A “10-page contract” might be a tidy 200 KB if it was generated as text — or a hefty 60 MB if it was printed, signed, and scanned. The reason: a scan is really just a stack of high-resolution images. The fix: compress the scanned PDF, which can shrink it by 80–95% in your browser, with no upload.

Why scans are so big

When you scan a document at 300 DPI (the typical default), each page becomes roughly a 2400 × 3300 pixel image. Saved as a JPEG inside a PDF, that’s around 2–8 MB per page depending on content. Multiply by 10 pages and you’re already at 20–80 MB. Color photographs and signatures push it higher.

Most of that data is invisible: 300 DPI is overkill for screen reading. The receiver of your scan almost always looks at it on a monitor, where 100–150 DPI is plenty. That’s the slack compression takes back.

Both presets work by downsampling the embedded images:

  • Recommended (≈150 DPI) — looks identical to most eyes on a monitor; still prints acceptably. Typical reduction: 60–90% smaller.
  • High (≈72 DPI) — clearly designed for screen viewing. Text remains crisp because text stays vector-perfect; only the underlying image is reduced. Typical reduction: 80–95% smaller.

If you OCR’d the scan beforehand (so the text is selectable / searchable), that text layer is preserved through compression — it lives separately from the image layer.

Before you compress

Two quick wins make a scan even smaller:

  • Delete pages you don’t need. Scanners often add blank separator pages or duplicate pages from accidental double-feeds. Pop the file through the Delete Pages tool first.
  • Fix sideways pages. Doesn’t change the size, but it makes the post-compression file actually useful. Rotate PDF handles per-page rotation.

When to keep more quality

If the scan needs to be printed (not just viewed on screen), or if it contains small print someone will need to enlarge, stick with Recommended. The High preset is great for emailing, attaching to forms, or uploading to portals — anywhere the file will be viewed on a screen.

Why doing this locally matters

Scans tend to contain exactly the documents you don’t want to give to a third party — signed contracts, IDs, medical paperwork, financial statements. This tool runs Ghostscript compiled to WebAssembly entirely inside your browser, so the scan never leaves your device. You can verify this in your browser’s Network tab: you’ll see the engine download, but never your file.

Step by step

  1. Open the Compress PDF tool.
  2. Drop the scanned PDF onto the upload area.
  3. Pick “Recommended” for most documents, “High” if you need it really small.
  4. Click “Compress PDF” and check the percentage saved.
  5. Download the smaller file.
Open the Compress PDF tool →

FAQs

Why are scanned PDFs so much bigger than regular ones?
A scanned PDF is really a stack of high-resolution images glued together. Each page might be a 2400 × 3300 pixel JPEG that the scanner saved at 300 DPI. Even a 10-page document can easily run 30–80 MB. Compression downsamples those images, which is why scans shrink the most.
How much smaller can I expect a scan to get?
Typically 60–90% smaller at "Recommended" and 80–95% at "High". A 50 MB scan often becomes 5–10 MB at Recommended and 2–5 MB at High. The tool tells you the exact before and after so you can see for the specific file.
Will the text in my scan still be readable?
Yes for normal reading. "Recommended" keeps images at roughly 150 DPI — equivalent to a decent magazine print — and "High" goes to about 72 DPI, which is screen resolution. If you need to zoom in to read small print, stick with Recommended.
Does compression damage searchable text in OCR'd scans?
No. Ghostscript downsamples the image layer but preserves the invisible text layer from OCR. The PDF stays searchable and selectable after compression.
Can I make a scan even smaller if it's already mostly black-and-white?
Not really, with this tool — it doesn't convert color scans to bitonal black-and-white (which would shrink them further but at quality cost). Re-scanning at lower DPI or as grayscale is the only meaningful next step, and even then High compression covers most ground.

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