Scanned PDFs are almost always larger than they need to be. The file size comes entirely from the images the scanner captures, and most scanners default to settings that produce much more data than the document actually requires. A few adjustments at scan time โ and a compression step afterward for files that are already too large โ bring sizes down dramatically without affecting legibility.
Match Scan Resolution to the Document's Purpose
Resolution (DPI) is the biggest lever for controlling scanned file size. Higher DPI captures more detail โ and creates a much larger file. For most documents, you're capturing far more detail than you need.
Practical DPI guidelines by purpose:
- Text documents for screen reading or email: 150 DPI is sufficient and keeps files compact
- Text documents that might be printed: 200 DPI produces clean prints without bloating the file
- Documents with photos or detailed graphics: 200-250 DPI for sharing, 300 DPI if you need print quality
- Archival scans of originals you want to preserve in detail: 300-600 DPI, accepting the larger file size
The jump from 300 DPI to 150 DPI reduces file size by roughly 75% because you're capturing one quarter as many pixels. For a standard text document, the output at 150 DPI is indistinguishable from 300 DPI at normal reading size.
Use Black-and-White Mode for Text Documents
Color scanning stores three values per pixel (red, green, blue). Grayscale stores one. Black-and-white (bitonal) stores one bit โ either black or white. For a document that's black text on white paper, bitonal scanning captures everything that matters and produces a file that can be 10 to 20 times smaller than a color scan at the same resolution.
Look for this setting in your scanner software under "Color Mode" or "Image Type." The options are usually Color, Grayscale, and Black & White or Monochrome. For contracts, forms, correspondence, invoices, and most business documents, black-and-white is the right choice. Switch to grayscale if the document has photographs or shading that matters.
Enable PDF Compression in Your Scanner Software
Many scanner applications have a compression option in the PDF settings that's disabled by default or set to a low level. Enabling it โ or increasing the compression level โ applies image compression at save time, which can cut file size by 50-70% from the raw scan without any quality visible difference.
The compression type matters too. For black-and-white scans, JBIG2 compression is very efficient and is specifically designed for bitonal document images. For grayscale and color scans, JPEG compression at 70-80% quality hits the best balance between file size and clarity for most documents.
Remove Blank Pages Before Saving
Scanners with automatic document feeders often pull through blank reverse sides of single-sided pages. Each blank page is still stored as a full image โ typically a large expanse of white with some scanner noise โ adding to the file size without adding content. A 20-page document with 10 accidental blank pages is 50% larger than it needs to be.
Some scanner software detects and removes blank pages automatically. If yours doesn't, a quick review after scanning and deleting blank pages before saving takes under a minute and makes a meaningful difference in the final file size.
Compress After the Fact if Needed
If you already have oversized scanned PDFs and can't re-scan, a PDF Compression tool handles the reduction without any quality you'd notice. Upload the scanned PDF, process it, and download a smaller version. For black-and-white text scans, compression tools can routinely reduce file size by 70-85%. For color scans, 40-60% reduction is typical at settings that keep text clearly legible.
The one thing to avoid is compressing a file that's already been compressed and is starting to show quality issues. Every compression pass on a JPEG image introduces more degradation. If a scanned PDF already looks rough, go back to the original scan rather than compressing further.
A Simple Default Setup for Everyday Scanning
If you scan documents regularly and want a single setting that works well for most situations: 200 DPI, grayscale, with PDF compression enabled. This produces files that are small enough to email easily, clear enough to read comfortably on screen and when printed, and good enough for OCR if you need to add searchable text later. Adjust from there for specific situations that need higher resolution or color.