You need Acrobat to do that, or enable doing that in Reader.
So the solutions are:
1. "Buy Acrobat"....which Adobe Acrobat ?....Stanard, Pro, etc (...obviously I'd like the most basic and cheapest...which one is that?)
2. "Enable doing that in the Reader"....how exactly am I supposed to do that?
All versions of Acrobat can add highlights, but only Acrobat Pro can allow
Reader users to do so.
However, if you have Acrobat, then that is moot.