How to Compress Scanned PDF Documents — Reduce Size While Keeping Text Readable

Scanned PDFs are the biggest compression opportunity. A 100MB scanned contract can become 10-15MB with zero readability loss. Here is how to compress scanned documents effectively.

Why Scanned PDFs Are 10-100x Larger Than Normal PDFs

PDF Type	Size Per Page	10-Page Document	Why
Scanned (300 DPI, color)	7-25MB	70-250MB	Each page is a full color photograph
Scanned (300 DPI, grayscale)	3-8MB	30-80MB	No color data, still a full image
Scanned (150 DPI, grayscale)	1-3MB	10-30MB	Lower resolution, smaller image
Digital PDF (text + charts)	50-200KB	500KB-2MB	Text stored as data, not images
Digital PDF (text only)	10-50KB	100-500KB	Minimal data per page

Scanned PDF Compression Strategy

1. Start with medium compression — this alone typically reduces by 60-80%
2. Check readability: open the result, zoom in on the smallest text. Readable? You are done
3. If still too large: consider grayscale conversion if the document has no meaningful color
4. If text is fuzzy: reduce compression level one step. The sweet spot is usually between medium and high
5. For very large documents (100+ pages): split into sections, compress each, then merge back

DPI for Scanned Documents

Use Case	DPI Needed	File Size Impact
Screen viewing only	100-150 DPI	Smallest — 1/4 to 1/9 of 300 DPI
Screen + occasional print	150-200 DPI	Good balance of size and quality
Print at original size	300 DPI	Standard scan quality
Legal/archival requirements	300 DPI	Required by most compliance standards
OCR text recognition	200-300 DPI	OCR accuracy drops below 200 DPI

Complete Scanned Document Workflow

1. Scan at appropriate DPI — 200-300 for most uses, not 600
2. Scan in grayscale unless color is specifically needed
3. Compress the PDF — reduce embedded image sizes
4. Need it searchable? Run OCR — adds text layer
5. Need specific pages? Split the PDF
6. Combining multiple scans? Merge, then compress the combined file
7. Sending via email? Check size against email limits