A scanned PDF that's 80MB for five pages of text is a common frustration. Scanning creates image files, not text files, and images — especially high-resolution color images — are inherently large. Three settings control almost all of the bloat, and adjusting them brings file sizes down by 80-90% without making the document less readable.
Resolution (DPI) Is the Biggest Factor
Every extra dot per inch multiplies the data stored. A letter-size page scanned at 600 DPI contains four times as many pixels as the same page at 300 DPI, and therefore roughly four times the uncompressed data. Most scanners default to 300 or even 600 DPI, which is designed for archiving or printing at large scale — far more than needed for a document you'll read on screen or email.
For screen reading and emailing: 150 DPI is enough. The text is sharp, the document is readable, and the file is a fraction of the size. For documents you'll print normally: 200 DPI. For archiving originals you might reproduce later: 300 DPI and accept the larger size. Go into your scanner's settings and change the DPI before your next scan.
Color Mode: Switch to Black-and-White for Text Documents
Color scanning captures three values per pixel (red, green, blue). Grayscale captures one. Black-and-white (bitonal) captures one bit — on or off. For a typed document on white paper, there's no useful color information to preserve. Scanning in black-and-white instead of color reduces file size by a factor of 10 to 20 with zero visible difference in the output.
In your scanner software, look for Image Type, Color Mode, or a similar setting. Choose "Black and White" or "Monochrome" for any document that's black text on white paper — contracts, letters, forms, reports. Switch to grayscale or color only for documents with photographs or colored elements that matter.
Enable PDF Compression at Save Time
Some scanner software saves PDFs with minimal or no compression by default. The raw image data goes straight in without being compressed. Look for a PDF quality or compression setting in your scanner app — even setting it to Medium instead of High can cut the output size in half for image-heavy scans.
Fixing Files That Are Already Scanned
If you already have oversized scanned PDFs, a PDF Compression tool handles them without rescanning. Upload the file, apply compression, download a smaller version. For a black-and-white text scan compressed this way, 70-85% size reduction with no visible quality loss is typical. WukongPDF's compression tool applies JBIG2 compression to bitonal content — the most efficient algorithm for scanned text pages — and JPEG compression to any photographic content.
For a definitive fix on documents you still have physical access to: rescan with the right settings (150-200 DPI, black-and-white, compression enabled). The results will be cleaner than post-processing an oversized scan.
Default Settings to Use Going Forward
Set these once in your scanner software and leave them: 200 DPI, grayscale (works for both text and simple graphics), compression enabled. This produces clean, reasonably small PDFs for the vast majority of scanning tasks. Bump to 300 DPI and color only when you specifically need it — don't leave the scanner at its high-quality defaults for everyday scanning.