100KB is a hard limit. It usually means the document will be rasterised and shrunk aggressively, so expect a visible quality trade-off rather than a free lunch.
Some older government and bank portals still cap single attachments at 100KB. If the form accepts a ZIP or multiple files, that is usually the better route than destroying the scan quality.
A one or two page text PDF can reach 100KB easily. A 20-page colour scan generally cannot, at any quality you would want to submit. The tool tells you what it actually produced.
Crop the page margins first, then remove pages you do not need, and compress last. Deleting a single blank or duplicate page often saves more than aggressive compression costs you in readability.
Long documents and colour scans carry more image data than 100KB can hold. Reducing page count or scan resolution first is the reliable fix.
Yes, in a modern mobile browser, though very large files are better handled on a computer.
No. There is no signup, no watermark and no usage limit.