Which size do I need?
Use the exact limit printed next to the upload button. Compress PDF takes any number in KB; the size pages above start with a common limit filled in and explain what usually fits in it. As a rough guide: 50 KB for a one-page certificate or ID proof, 100 to 200 KB for a mark sheet or a few scanned pages, 500 KB to 2 MB for long or colour documents.
How the PDF gets smaller
First the PDF is rewritten losslessly, which keeps text selectable and is often enough for typed documents. Only if that is still too big are the pages redrawn as images at the sharpest quality that fits, with an optional black and white mode for scans of printed paper. Every page is kept, in order, at its original size.
What happens to your PDF
Everything runs in your browser: the PDF is not uploaded to DocFig or anyone else. Files up to 25 MB and 100 pages work on phones and computers. If the PDF is locked, remove the password first. For photos and signatures that must be JPG, use the image tools instead.