A scanned PDF of a five-page letter shouldn't be 80MB. But it often is, and the reason almost always comes down to how the scan was set up rather than anything about the document itself. The file size of a scanned PDF has nothing to do with how much text is on the page โ it's determined entirely by the image data the scanner captured. Understanding that distinction points directly to the fix.
The Scanner Resolution Was Set Too High
This is the most common cause of oversized scanned PDFs by a wide margin. Scanner resolution is measured in DPI (dots per inch). The higher the DPI, the more image data is captured โ and the larger the resulting file. Many scanners default to 300 or 600 DPI, which is appropriate for archiving documents you might need to print in large format or zoom into closely. For a standard text document you're sharing by email or uploading to a portal, it's overkill.
At 300 DPI, a single A4 page in color produces roughly 25MB of raw image data before any compression. Multiply that by a 10-page document and you're starting at 250MB before the PDF even gets saved. Most scanners apply compression automatically, but the starting point is still a lot of data.
For text-only documents, 150 DPI is enough for clean, readable output. For documents with photos or detailed diagrams, 200 DPI is a reasonable ceiling for sharing purposes. Save 300+ DPI for genuine archiving needs where you might need to reproduce the document at full quality later.
Color Scanning Instead of Grayscale or Black-and-White
Color scans store three channels of data for every pixel โ red, green, and blue values. A grayscale scan stores only one. A black-and-white (bitonal) scan stores one bit per pixel โ either black or white, nothing in between. The difference in file size is significant: a color scan of a plain text document can be 10-20x larger than the same document scanned in black-and-white.
If your document is black ink on white paper โ which describes most business documents, contracts, and forms โ there is no benefit to scanning in color. The scan will capture the slight cream color of the paper, the gray shadows near the staple holes, and the subtle variation in ink density, none of which you need. Switching to black-and-white or grayscale mode in your scanner settings cuts file size dramatically.
No Compression Was Applied at Save Time
Some scanner software saves scanned images with minimal or no compression. The scanner captures the raw image data and writes it to the PDF almost as-is, resulting in a file that's unnecessarily large for what it contains. This varies by scanner and software โ some apply aggressive compression by default, others barely touch it.
If you already have an oversized scanned PDF, running it through a PDF Compression tool is often the fastest fix. A good compression tool applied to a scanned PDF can reduce the file size by 60-80% with no visible change in quality โ especially for black-and-white text documents where the image data compresses extremely efficiently.
The Scan Includes Unnecessary Pages or Blank Pages
Scanners with automatic document feeders sometimes pull extra blank pages through, especially if the document has a back side that's empty. Each blank page still gets stored as a full image in the PDF, contributing to file size without adding any useful content. A 20-page scan with 6 accidental blank pages is 30% larger than it needs to be.
After scanning, quickly review the PDF and delete any blank or duplicate pages before saving or sharing. Most PDF tools let you remove individual pages โ it takes about 30 seconds and can meaningfully reduce the final size.
How to Reduce the Size of an Existing Scanned PDF
If the scan is already done and the file is too large, you have two main options: compress it or re-scan with better settings. Re-scanning is the cleaner solution if you still have access to the physical document and want the best possible result. But if you just need to get the file under a size limit quickly, compression works well.
WukongPDF's PDF Compression tool handles scanned PDFs well because it optimizes the embedded images directly โ which is exactly what a scanned PDF is made of. Upload the file, choose your compression level, and download the result. For a typical scanned text document, the output is usually 70-85% smaller than the original with no visible quality loss.
Scan Settings to Use Going Forward
A few changes to your scanner settings will keep files manageable from the start:
- Text-only documents: black-and-white mode, 150 DPI
- Documents with photos or graphics: grayscale or color, 200 DPI
- Archiving originals you may need to reproduce: 300 DPI, color if applicable
- Always enable PDF compression in your scanner software if the option is available
Scanned PDFs don't have to be massive. The right settings at scan time keep files small without sacrificing the quality you actually need โ and a compression pass after the fact handles anything that slipped through.