Limits of 100 KB are common on government, exam and scholarship portals for documents such as certificates, caste or income proofs and mark sheets. A one or two page scan can usually reach 100 KB; a long scanned file may need to be split first.
Most can. The tool tries stronger settings until the file is 100 KB or less. If a long scanned PDF still will not fit, split it with Split PDF by size, or remove pages you do not need, and try again.
The tool first tries to shrink the file without any loss, then picks the highest quality that still fits your target. To reach small sizes, pages are saved as images, so the text stays readable but may no longer be selectable. Check the result before you upload it.
No. It is compressed inside your browser, so it never leaves your phone or computer.