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How to reduce the size of a scanned PDF

By Mehroz Shabir · Updated 15 October 2026

A few pages scanned with a phone can easily reach 10–20 MB. Here is why, and six ways to bring a scanned PDF under an upload limit while keeping it readable.

Why scanned PDFs are so big

A scanned PDF is really a set of photos, one per page. Phone cameras and scanners save each page with far more detail than you need to read it, often in full colour. A PDF made in Word, by contrast, stores text as letters, so ten pages of plain text can be under 100 KB while ten scanned pages can be 15 MB.

1. Compress it to the exact size

The fastest fix. Our compressor re-saves each page as a smaller image and keeps the sharpest version that fits your target. Pick the limit you need: 200 KB, 500 KB, 1 MB, 2 MB, or type any size into Compress PDF.

2. Remove pages you don't need

Blank backs of pages, cover sheets and duplicate copies all add size. Use Split PDF to keep only the pages the form asks for. Halving the pages roughly halves the size, and leaves more room for quality on the pages that matter.

3. Scan in greyscale

Unless the form asks for colour, scan in black and white or greyscale. Colour stores three times as much information per pixel. Most phone scanner apps and office scanners have a greyscale setting.

4. Use a sensible resolution

150–200 dpi is plenty for reading and printing documents. Scanners set to 600 dpi produce files several times larger, with detail no one will see. If you photograph pages with a phone, the compressor reduces the resolution for you.

5. Crop and straighten before making the PDF

Tables, fingers and shadows around a page are stored as detail too. Take the photo straight from above, in daylight, on a dark plain surface, and crop to the edges of the paper. Clean, evenly lit pages compress far better.

6. Split one big file into several

If a portal allows several uploads, split a long document into parts with Split PDF and compress each part. If it wants a single file, do the opposite: join everything with Merge PDF first, then compress the merged file once.

Roughly how many scanned pages fit

These are rough figures for greyscale or simple colour pages. Pages full of photos need more space per page.

One trade-off to know

After compressing a scanned PDF, the pages are images, so text can't be selected or searched. That's normal for scans and fine for forms. If you need editable text, run a page through Image to Text (OCR), or convert a text-based PDF with PDF to Word.

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