How to Compress a Scanned PDF Online for Free — Reduce File Size Fast

Scanned PDFs are almost always massive. A 10-page document scanned at 300 DPI can easily reach 20–50MB — far too large for email attachments, portal uploads or efficient storage. The good news is that scanned PDFs compress extremely well. Here is how to reduce a scanned PDF from 30MB to 2MB while keeping it completely readable.

Why Are Scanned PDFs So Large?

When you scan a document, the scanner captures each page as a high-resolution image — typically 300 DPI (dots per inch) or higher. This image is then embedded inside a PDF container. A single A4 page scanned at 300 DPI in colour produces an image of approximately 2480 × 3508 pixels — about 25 million pixels of data. Multiply that by 10 pages and you have 250 million pixels of data in one file.

Text-based PDFs created from Word documents are tiny by comparison because they store text as characters and instructions, not as images of text.

How Much Can a Scanned PDF Be Compressed?

Scanned PDFs compress dramatically well because image compression algorithms are highly effective on document content. Typical results:

• Colour scanned document: 80–90% size reduction possible
• Black and white scanned document: 85–95% size reduction possible
• Mixed content (text + photos): 70–85% size reduction

A 30MB scanned contract can typically be reduced to 2–4MB while remaining completely legible on screen.

Choosing the Right Compression Level

• Screen (72 DPI): Maximum compression — reduces to smallest possible size. Text remains readable on screen. Not suitable for printing. Best for email attachments, WhatsApp, portal uploads where only screen viewing is needed.
• eBook (150 DPI): Balanced — good quality on screen and tablets, moderate compression. Best for documents shared for digital reading.
• Print (300 DPI): Minimal compression — preserves print quality. Use when the recipient needs to print the document at full quality.

Step-by-Step: Compress a Scanned PDF

1. Open the PDF Compressor
2. Upload your scanned PDF (up to 25MB)
3. Select Screen compression for maximum reduction, or eBook for balanced quality
4. Click Compress PDF
5. Check the before and after file sizes shown
6. Download your compressed PDF

Tips for Even Smaller Scanned PDFs

• Scan in black and white: If the document has no important colour information, scanning in B&W produces files 3–5x smaller than colour scans before compression
• Reduce scan DPI at source: For documents that will only be viewed on screen, scanning at 150 DPI instead of 300 DPI produces much smaller source files
• Split and compress: For very large multi-page scans, split into sections using our PDF Splitter, compress each section, then merge back with our PDF Merger
• Remove blank pages: Scanners often capture blank backs of pages — remove these with our Remove PDF Pages tool before compressing

When Compression Is Not Enough

If your scanned PDF is too large to upload at all (our limit is 25MB), try these steps first:

1. Re-scan at lower DPI (150 DPI is sufficient for most documents)
2. Scan in black and white instead of colour
3. Scan fewer pages at a time
4. Use your scanner's built-in compression settings