Getting a PDF under 100 KB without ruining it
Hitting an exact size limit is a balancing act: too little compression and the upload is rejected, too much and the scan becomes unreadable. This page approaches 100 KB in steps and stops at the first quality level that fits, so you keep the best possible image for that size.
If your document is mostly text, expect a large reduction with no visible change at all. If it is a photographed page, expect softer detail — shooting the page in better light usually helps more than heavier compression.