A PDF that will not send is nearly always a PDF full of pictures. Understanding that makes the fix obvious and tells you how much you can expect to gain.
Compress it. Scanned and image-heavy documents usually drop dramatically. If it is still over the limit, convert to greyscale or split it into parts.
It depends entirely on what is in it. A scan can often lose most of its size with no practical loss of legibility; a document that is already just text has almost nothing to give.