The best scan settings for documents: resolution, colour and file size

The short answer

Resolution: what DPI really changes

DPI — dots per inch — is how many pixels the scanner captures for every inch of paper. An A4 page is about 8.3 × 11.7 inches, so at 300 DPI it becomes an image of roughly 2,480 × 3,508 pixels. A US Letter page at 300 DPI is 2,550 × 3,300.

The catch is that the pixel count grows with the square of the resolution. Doubling from 300 to 600 DPI doubles the width and the height, so the page has four times as many pixels and the file is several times larger, while ordinary text looks no different on screen.

• 150 DPI is readable on screen but small print and thin lines start to soften.
• 200 DPI is enough for most typed documents you'll read on screen or print once.
• 300 DPI is the usual choice for documents you want to keep, print well or run text recognition on.
• 600 DPI is for fine detail: tiny print, technical drawings, or photos you'll crop and enlarge.

Colour mode: the biggest size decision

Black and white (sometimes called 1-bit, line art or text mode) stores each pixel as pure black or pure white. Files are tiny and printed text looks crisp, but pencil, light signatures, stamps and photos can break up or vanish.

Grayscale stores shades of grey. It keeps faint handwriting, signatures and stamps readable and handles pages with pictures, at a moderate size. For most paperwork it's the best balance.

Colour stores three values for every pixel instead of one, so before compression it's roughly three times as much data as grayscale. Use it when the colour carries meaning: coloured stamps, highlighting, charts, photos, or documents such as ID cards where an official may expect to see the original colours.

Tip: If a form or portal says 'colour scan required', follow it even when grayscale looks fine — it's their rule, not a quality judgement.

Phone or scanner?

A flatbed or a scanner with a document feeder gives even lighting, square pages and a consistent resolution, and a feeder is far quicker for long stacks. If it only scans one side, you can still scan double-sided documents: scan the fronts, flip the stack, scan the backs, then put the pages in order with Interleave PDF.

A phone is fine for a few pages, especially with a scanning app that finds the page edges and flattens the perspective. Lay the page flat on a dark surface, use bright, even light without your shadow on it, and hold the phone parallel to the paper. Phone cameras capture far more pixels than a page needs, so the photos are large: that's the main reason a PDF of phone scans grows to tens of megabytes.

If your phone saves photos rather than a PDF, JPG to PDF combines up to 200 images into one document, one page each, on A4, US Letter or each image's own size. JPG and PNG images go in exactly as they are, without being re-compressed; HEIC, WebP and other formats are converted to high-quality JPG on your device first. That keeps full quality — and full size — so compress the result afterwards if it needs to be smaller.

Getting the file size down afterwards

If you've already scanned at a high resolution, you don't have to scan again. Compress PDF re-encodes the page images as JPEGs and scales down any that are bigger than needed. The Recommended level limits images to 1,800 pixels on the longest side — on a full A4 page that's roughly the detail of a 150 DPI scan. Light allows 2,600 pixels (about 220 DPI on A4) and Strong 1,200 pixels (about 100 DPI). For a hard limit, type a target size such as 1,024 KB instead.

Two things to know. Black-and-white (1-bit) scans are left as they are: they're already very compact, so there's usually little to gain. And converting an existing PDF with Grayscale PDF is not a way to make it smaller. It re-renders every page as a grey image at 150, 200 or 300 DPI, which removes colour reliably but turns any real text into pictures — text-only PDFs usually get bigger, and scan-heavy PDFs often stay about the same size. Choosing grayscale on the scanner in the first place is the better way to save space.