Compress PDF to 20 KB

Limits this small were designed for images. Indian recruitment forms, including those of the Staff Selection Commission, have long asked for the scanned signature as a JPEG of roughly 10–20 KB, and some state and university portals reuse that field size for small PDFs: a signed declaration, an undertaking or a thumb-impression sheet. The portal rejects anything with more bytes than the number printed next to the button.

At 100 KB the tool trims a scan until it fits. At 20 KB it has to discard most of the page's pixels, so the result depends on the page. Typed text, plain paper and dark, sharp ink are cheap. Shadows, a tinted background, a crumpled sheet or a photo stapled to the corner are expensive.

A page scanned at 300 DPI is about 2480 × 3508 pixels. At 72 DPI the same page is 595 × 842 pixels, which is also the size of an A4 page in PDF points, the unit the format has used since Adobe published it in 1993. Ghostscript's /screen preset downsamples to 72 DPI for the same reason, and it is where a 20 KB budget starts to work for a full grayscale page.

With Target size set, UltKit re-encodes every image and checks the result against 20 KB. It lowers JPEG quality first, down to Lowest image quality, then reduces resolution. Remove metadata and Subset fonts are on by default and matter more here than anywhere else on the ladder: a few kilobytes of XMP data or a fully embedded font can be a fifth of the allowance. The guide to scanning documents to PDF covers scanner app settings that keep a page small from the start.