The photo belongs to the photographer, not to you. If you signed anything, including a money receipt, there is most likely a rights waiver attached that allows the photographer to use the photo as he chooses.
did you see any options that applied directly to images? Sorry I can't be more specific in instructions. I work with Acrobat for high end printing, and my version has capabilities others don't, but I can't remember what is built in and buried vs what is left out of free versions.
In true Acrobat app, there are several ways to reduce file size ... and the best is called Reduce File Size. Can't speak to other PDF apps, but most will have a searchable help menu.