A ten-page scan straight from a phone app can easily reach 30 or 40 MB. That is because each page is saved as a full-resolution photograph, often in colour, even when the page is plain black text on white paper. The good news: scanned PDFs compress better than any other kind.
Start at the Source
The biggest savings happen before the PDF exists.
- Resolution: set your scanner or scanning app to 150–200 DPI. 600 DPI is for archives and photo restoration, not form uploads.
- Colour mode: switch to greyscale or "document" mode for text pages. Colour triples the data per pixel.
- Cropping: remove the table or floor around the page. Many apps auto-detect edges.
- One page per image: avoid photographing two pages at once and then cropping — it wastes resolution on the gap.
Then Compress the PDF
- Open Compress PDF.
- Upload the scanned file.
- Choose ebook for clear results, or screen if you have a strict size limit.
- Download, then zoom to 150% to confirm the text is readable.
Typical results on a colour phone scan: 30 MB down to 3–5 MB at ebook, and under 2 MB at screen.
If You Photographed Pages as JPGs
Convert them into one document with JPG to PDF, put them in the right order, then compress. If the photos are enormous, shrinking them first with Image Compress gives even smaller results.
Make It Searchable Too
A compressed scan is still an image. If you need to search it or copy text, run OCR PDF. Our guide to extracting text from scanned PDFs explains how to get accurate results.
Common Problems
- Text becomes fuzzy: you compressed too hard. Go back to the original and use ebook instead of screen.
- Grey background looks dirty: improve lighting when scanning; compression amplifies shadows.
- Pages are sideways: fix them with Rotate PDF before compressing.
Summary
Scan at a sensible resolution, use greyscale for text, then compress. Those three habits turn unmanageable scans into files any portal will accept.
Scanner App Settings Worth Changing
Most phone scanning apps default to high-quality colour. Look for these options before you scan:
| Setting | Change it to | Effect |
|---|---|---|
| Colour mode | Greyscale or "Document" | Often halves the size |
| Quality / resolution | Medium or 150–200 DPI | Large saving, still readable |
| Page size | A4 | Avoids huge borders |
| Auto-crop | On | Removes background |
| Enhance / clean-up | On for text | Whiter paper compresses better |
Colour vs Greyscale: A Quick Example
A three-page colour scan of a bank statement might be 12 MB. The same pages in greyscale at 200 DPI are around 3 MB before compression, and under 1 MB after compressing at the ebook level — with every figure still easy to read.
When You Should Keep Colour
Keep colour for photographs, documents where stamps or signatures must show their ink colour, maps and charts that use colour coding, and any file an office has explicitly asked to be "in colour." In those cases, reduce the resolution instead of removing colour.
Keep the Original
Always save the uncompressed scan somewhere safe. If an office later asks for a clearer copy, you can produce one without rescanning the paper.
