500 KB: room for colour and detail
Half a megabyte is a comfortable limit for most single documents. A colour certificate keeps its seal and its fine print, a passport-style photo page keeps the face recognisable, and a typed document of many pages usually passes with no change at all.
The size this page targets is 500,000 bytes, a little under 500 KB counted the other way, so the copy passes whichever way a portal counts.
Phone scans
Scanner apps on phones often save each page as a large, high-resolution picture, so a three-page scan can be several megabytes. That is mostly detail no form needs. Redrawn for 500 KB, the same pages usually keep sharp text and readable stamps in colour. The preview shows page 1 as it will look, and the result line gives the exact size before you download.
- Use the scanner app's document mode, which straightens and crops the page.
- Scan in good, even light; shadows cost bytes and hide detail.
- Save as PDF in the app, then compress the PDF here. If the app offers a quality or size setting, a medium setting is a good starting point.
When to choose black and white
For colour documents, leave it off: at 500 KB the colour version normally looks good. Choose it for long scans of plain printed pages, where it keeps every page crisp and leaves plenty of room. A black and white copy turns stamps and photographs black and white too, so it is not the right choice for an identity document with a photo.
Typed PDFs stay as text
A PDF made from Word, a website or an online form contains real text and fonts. Those are small, so the lossless rewrite almost always brings such a file under 500 KB without touching how it looks, and you can still select and search the text in the copy.
Other sizes
For a tighter limit, compress PDF to 300 KB; for larger files, compress PDF to 1 MB. Any other number works on compress PDF.
Built on two open-source projects that run in your browser: qpdf, compiled to WebAssembly, for the lossless rewrite, and Mozilla's PDF.js to redraw pages when that is not enough.