Gmail stops at 25 MB, Outlook often at 20 MB, and plenty of corporate mail servers cut off at 10 MB. Scanned documents hit those ceilings constantly.
Attachment limits usually apply to the encoded size, which is roughly a third larger than the file on your disk. A 19 MB PDF can still bounce off a 25 MB limit, so leave yourself room rather than aiming exactly at the number.
If the document is mostly scanned pages, compression will typically take 60 to 80 per cent off. If it is a text document exported from a word processor, it is already efficient and you will save very little.
When a document genuinely will not fit, a link is better than a squeezed file. Compressing a 60 MB scan down to 20 MB will cost real legibility.
Because it was already text rather than images. There was nothing heavy in it to remove.
Some, yes. Balanced is set to stay clearly readable in print, and you see the result before saving.
No. It is compressed by your own browser.