What Scan Resolution Should You Use for PDFs?
Scan resolution determines whether a scanned PDF is readable, OCR-accurate, and correctly sized. Learn the right DPI settings for different document types and use cases.
The resolution you scan a document at directly determines the quality of the resulting PDF — both the visual clarity and the accuracy of any OCR processing. Too low and text becomes blurry, OCR fails, and the document looks poor when zoomed. Too high and file sizes balloon needlessly. The right choice depends on the document type and what you'll do with the PDF.
Resolution Guidelines by Use Case
- General document archiving (for reading only): 200 DPI — adequate for standard body text, readable at normal zoom, modest file size
- Document archiving with OCR: 300 DPI minimum — the industry standard for accurate OCR on standard print sizes (10-12pt body text)
- OCR for small text or fine print: 400-600 DPI — footnotes, legal fine print, compressed type need higher resolution for reliable character recognition
- Line art and technical drawings: 600-1200 DPI — fine lines and precise details need high resolution; these are typically smaller file sizes than photos even at 1200 DPI
- Photographs: 300-600 DPI — for archival photo scanning; 300 DPI is usually sufficient for documentary use
Why 300 DPI Is the OCR Threshold
OCR engines are typically trained on images that assume characters are at least 20-30 pixels tall. At 300 DPI, a 12pt character (1/6 inch) is 50 pixels tall — well above the threshold. At 150 DPI, the same character is 25 pixels — marginal. At 72 DPI, it's 12 pixels — below the practical OCR floor. Character distinctions between similar letters (i vs l, 0 vs O, rn vs m) that a human can easily read become ambiguous to OCR below 200 DPI. This is why 300 DPI is the widely cited minimum for reliable OCR.
Color, Grayscale, or Black and White?
For text documents with no images: Black and White (1-bit) scanning at 300 DPI produces the smallest file sizes and excellent OCR results for clean documents. For mixed documents with photos and text: Grayscale preserves tone while keeping files smaller than color. Color scanning is needed for: documents with color-coded information (charts, diagrams, highlighted text), photographs, and any document where color carries meaning. Color scans at the same DPI produce 3× larger files than grayscale.
File Size Reality Check
A typical US Letter (8.5×11") page scanned at different settings and compressed to PDF with moderate JPEG compression: 150 DPI color ≈ 150-300 KB; 300 DPI color ≈ 400-800 KB; 600 DPI color ≈ 1.5-3 MB; 300 DPI grayscale ≈ 100-250 KB; 300 DPI B&W ≈ 20-80 KB. A 100-page scanned report at 300 DPI color will be 40-80 MB — consider whether the color information is necessary, or if grayscale at 300 DPI (5-25 MB) would serve the use case better.
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