March 22, 2026
You scan a 15-page document at the office, open the resulting PDF, and check the file size: 87MB. For 15 pages. That is bigger than most video clips people share online.
The scanner is not broken. Scanned PDFs are inherently large, and once you understand why, you will know exactly how to deal with them.
Why Are Scanned PDFs So Huge?
A regular PDF -- the kind you export from Word or Google Docs -- stores text data and layout instructions. A single page might be 50KB. Scanned PDFs are fundamentally different. Every page is a full-resolution photograph of the paper.
A scanner set to 300 DPI color mode produces pages that are roughly 8MB each. Scan 20 pages and you are looking at 160MB of raw image data. Even with basic compression inside the PDF container, the final file easily lands in the 50-100MB range.
To put it in concrete terms: 300 DPI means 300 pixels per inch. An A4 sheet is about 8.27 by 11.69 inches, giving you an image of roughly 2481 by 3507 pixels. In color (3 bytes per pixel), that is 26MB of uncompressed data per page.
What You Can Do Before Scanning
If you have not scanned yet, or if you can redo the scan, adjusting your scanner settings is the most effective way to reduce file size at the source:
- Lower the DPI: 150 DPI is plenty for archiving and on-screen reading. Reserve 300 DPI for documents you plan to print or zoom into
- Use grayscale: if the document is a standard black-text-on-white-paper affair (contracts, invoices, forms), grayscale cuts the file size to roughly one-third of color
- Use black-and-white mode: for pure text documents, true black-and-white (not grayscale) produces the smallest files
- Match the scan area to your paper: make sure you are not scanning extra white margins
Switching from 300 DPI color to 150 DPI grayscale can reduce file size by about 90% for the same document.
How to Shrink a PDF That Is Already Scanned
More often than not, the PDF has already been scanned. Someone sent it to you, or you scanned it weeks ago and do not want to redo it. That is where post-scan compression comes in.
With PDF Under's compress tool:
- Drag your scanned PDF into the upload area
- Set your target size (for example, 8MB for email or 1.5MB for a government portal upload)
- Click compress
The tool works by re-rendering each page image at an optimized DPI and JPEG quality level until the total file size hits your target. Since scanned PDFs are entirely made of images, this compression approach is particularly effective on them.
Does Compression Affect Text Recognition?
If your scanned PDF has an OCR (optical character recognition) layer, that text data will not be preserved after compression. However, most scanned PDFs do not have OCR to begin with -- they are just images.
As for readability: as long as you are not compressing to an extreme degree (say, from 80MB down to 500KB), the text on the scanned pages remains legible. Compressing to 10-20% of the original size is a reasonable range. At that level, text clarity holds up well for normal viewing.
A Real-World Example
Say you have a 20-page scanned contract, 300 DPI color, weighing in at 65MB. Your client's file upload system has a 10MB limit.
Drop the file into PDF Under, set the target to 8MB (leaving some headroom), and the tool handles the rest in about 30 seconds. Open the compressed PDF: text is still sharp, stamps are recognizable, signatures are legible. You would only notice quality differences if you zoomed past 200%.
For standard business use, that level of quality is more than sufficient.