PDF GUIDE
Understanding Scanned PDF Compression
Scanned PDFs are some of the largest PDFs people work with. They are also some of the most rewarding to compress, because a small change in image quality can produce a large change in file size.
Why scanned PDFs are large
When you scan a page, the scanner captures the page as an image. Every letter, line and white space becomes pixels. A normal electronically generated PDF can store the same sentence as a few bytes of text; a scanned PDF stores the whole page as an image, which may be hundreds of kilobytes or even megabytes.
Multiply that by tens or hundreds of pages and the file size grows quickly. A 100-page scanned report at 300 DPI can easily exceed 50 MB.
How scanned PDF compression works
Compression tools cannot turn the scanned image back into text, but they can reduce the amount of data needed to store the image. They do this by:
- downsampling images to a lower resolution;
- using stronger image compression; and
- removing redundant image data.
The result depends on the content. A scanned page with mostly black text on white paper can usually be compressed more aggressively than a scanned page with photographs or gray shading.
Watch out for text quality
The biggest risk when compressing scanned PDFs is making text hard to read. Aggressive compression can blur small fonts, merge thin lines, or introduce artifacts around characters. This is especially important for:
- contracts and legal documents;
- receipts and invoices with small print;
- medical or technical records;
- anything that may need to be printed.
If the PDF has OCR text
Some scanned PDFs include an invisible text layer created by optical character recognition (OCR). This lets you select and search the text even though the page is an image. Compression that changes the visible image should not affect the OCR layer, but it is still wise to inspect the compressed file before archiving the original.
Recommended workflow
- Start with the Ebook / balanced preset in QuakeStar's PDF Compressor.
- Open the compressed file and read a few pages at 100% zoom.
- If the text is still sharp but the file is still too large, try Screen / smallest file.
- If the text becomes blurry, go back to Printer / high quality or keep the original.