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Scanning a document seems straightforward until you end up with a file that is either too blurry to read or too large to email. The settings you choose — resolution, color mode, and compression — determine both the visual quality and the practical usability of the resulting PDF. Get them right and you have a crisp, compact, searchable document. Get them wrong and you have either a pixelated mess or a file that eats up storage. This guide explains what each setting does and which combination to choose for different real-world situations.
DPI stands for Dots Per Inch, and it measures how much detail your scanner captures per unit of document area. Higher DPI means sharper images and larger files. Here are the three most practical settings and when to use each:
150 DPI: Screen reading and basic archiving
At 150 DPI, standard-sized text (10pt and above) is legible on screen, and file sizes stay small — typically under 100 KB per page in black-and-white. This setting works well when you need a quick digital copy of a document that you will read on-screen and never print. It is not suitable for OCR, small text, or anything you intend to print, because the resolution is too low to reproduce fine detail accurately.
300 DPI: The universal standard — right for most situations
300 DPI is the industry-standard resolution for document scanning, and the default setting on most scanners, for good reason. It accurately captures text as small as 8pt, produces high OCR accuracy, and yields files that are large enough to look good but small enough to be practical. A typical A4 page scans to roughly 50–200 KB in black-and-white or 300 KB–1 MB in color at 300 DPI. For contracts, forms, reports, correspondence, or any document destined for OCR, 300 DPI is the safest default.
600 DPI: When detail really matters
Use 600 DPI when your document contains very fine detail that must be preserved: photographs, artwork, engineering drawings, technical diagrams, very small text (below 8pt), or delicate signatures and stamps. The trade-off is significant — a color A4 page at 600 DPI can run from 5 to 20 MB, making multi-page documents unwieldy. Reserve this setting for situations where image fidelity genuinely justifies the storage cost.
Above 600 DPI: Rarely necessary outside of specialized fields like archival preservation, medical imaging, or professional photography reproduction. Higher resolution does not make standard text any clearer — it only produces much larger files.
Color mode affects both file size and which details in your document are preserved. Choose the wrong mode and you lose information; choose one that is richer than necessary and you waste storage.
Black-and-White (Binary / 1-bit): In this mode, every pixel is either pure black or pure white. This produces the smallest files by far, and for clean, printed text it can actually look sharper than grayscale because contrast is maximized. Best for: typed documents, clearly printed contracts and forms, anything that is strictly black ink on white paper. Avoid it for: handwritten documents, anything with photos or gray-tone imagery, light-colored stamps, or printed backgrounds — these details disappear or become unrecognizable when forced into a two-tone image.
Grayscale (8-bit): Grayscale preserves 256 levels of tone between black and white, capturing pen pressure in handwriting, pencil sketches, photograph shadows, and the range of gray tones found in most real-world documents. This is the most versatile mode. It handles handwritten notes, mixed text-and-image reports, newspaper clippings, and receipts well. File sizes are typically one-third to one-quarter of equivalent color scans, making it a good balance between fidelity and storage efficiency.
Color (RGB / 24-bit): Color mode captures the full color information in a document. Use it when color carries meaning — color-coded charts, highlighted contract clauses, color logos, documents with colored stamps or seals, or any material where the color itself conveys information you need to preserve. Color scans produce the largest files and should be avoided when the document content is genuinely monochrome.
After scanning, most software compresses the image to reduce file size. Understanding the main compression options helps you avoid unexpected quality loss:
JPEG compression (lossy): JPEG achieves high compression ratios by discarding some image data. For photographs and color documents, medium-quality JPEG (70–85%) is usually indistinguishable from the original and dramatically reduces file size. However, for text documents, JPEG introduces “ringing” artifacts — subtle halos around character edges that blur fine text and reduce OCR accuracy. Avoid JPEG compression for documents where text clarity is the priority.
PNG compression (lossless): PNG retains all image data without any quality loss. It compresses less efficiently than JPEG but ensures that fine details, including the edges of text characters, are perfectly preserved. This is the safer choice for documents heading into OCR processing or long-term storage.
CCITT and JBIG2 (PDF-native black-and-white compression): These compression formats are built into the PDF specification specifically for black-and-white document images. CCITT is lossless and extremely efficient for clean printed text; JBIG2 is even more efficient (though it can be slightly lossy at aggressive settings). If your scanning software offers these options for black-and-white PDFs, enable them.
PDF/A for long-term archiving: If the document needs to remain readable for years or decades, consider saving as PDF/A, the ISO-standardized archival variant of PDF. It prohibits features that depend on external resources (like embedded scripts or links to external fonts), ensuring the file remains fully self-contained and readable far into the future. This format is recommended for legal documents, government records, and any document with permanent significance.
The gap between a dedicated flatbed scanner and a smartphone camera has narrowed considerably, but it has not disappeared entirely. Here is a realistic comparison:
Where dedicated scanners still win: A flatbed scanner uses a precision CCD or CIS sensor illuminated by a controlled, even light source. There is no hand shake, no variable ambient lighting, and no lens distortion. At equivalent DPI settings, a flatbed scanner typically produces sharper text edges and more accurate color reproduction. For delicate materials — old photographs, thin pages, film negatives — a scanner also handles the physical document more gently and evenly. For high-volume scanning (more than 50 pages), a scanner with an automatic document feeder (ADF) saves enormous amounts of time compared to manual phone scanning.
Where smartphone scanning is genuinely good enough: For standard document archiving, email attachments, and OCR use cases, a flagship smartphone camera in good lighting, processed through a modern scanning app with perspective correction and image enhancement, produces results that are functionally indistinguishable from an entry-level flatbed scanner. Convenience is the real differentiator — your phone is always with you.
The practical recommendation: Use your phone for everyday document capture. Invest in a scanner if you regularly need to archive photographs, process large batches of documents, or require archival-quality reproductions of irreplaceable materials.
If the file is confidential, the safest route is the one where it never leaves your computer. PDF Compress runs entirely in the browser: the file is read from your disk, processed in memory, and written back — nothing is sent to a server.
Do it in your browser — the file never leaves your device: PDF Compress