Messaging apps cap document attachments, and a phone scan will exceed that cap surprisingly often.
There is a point where more compression stops saving meaningful space and starts making the document hard to read. If you are already at the smallest setting and still over the limit, sending two files is a better answer than sending one unreadable one.
Documents sent through a messaging app are usually passed through untouched, unlike photos. That is good, because your compression choices survive, and it means the file size you produce is the file size that has to fit.
Scans compress far more than exported documents. If your PDF came from a word processor and is still too large, something inside it is an image, and replacing that is more effective than compressing the whole file.
It varies by app and changes over time. Most sit somewhere between 100 MB and 2 GB for documents, though older versions are much stricter.
On Balanced, yes. On Smallest, check it before sending.
No.