Why Scanned PDFs Are So Large, and How to Shrink Them
A scanned page is a photograph of paper, and photographs are the heaviest thing a PDF can contain. That is the whole explanation — and it also tells you exactly what to change.
The numbers
One A4 page scanned in colour at 300 DPI is about 8 MB before compression. Ten pages: 80 MB. The same ten pages in greyscale at 150 DPI, recompressed as JPEG, land near 2 MB — and remain perfectly readable.
The information did not change. The redundancy did.
Three settings that matter
- Grayscale instead of colour. Scanner colour casts are noise; removing them cuts the data by two thirds and usually improves legibility.
- 150 DPI instead of 300. Text is legible at 150 DPI on screen and on a home printer. Reserve 300 for documents that will be OCRed or printed professionally.
- Recompress as JPEG at good quality. Scanners often store images losslessly, which is wasteful for photographs of paper.
Order of operations
- Rescan if the original scan was poor — no compression fixes a blurred page.
- Run OCR before compressing if you need searchable text. Compression artefacts reduce OCR accuracy.
- Compress last, and keep the pre-compression copy.
- Check the smallest text after compressing. That is what suffers first.
Privacy
Scanned documents are the most personal files people handle — IDs, bank statements, medical letters. Compressing them through an online service means uploading the full, unredacted original.
A browser-based compressor does the work on your device. Nothing is uploaded, and the compressed copy is the only file that leaves it.