How can I change the font used in Preview PDF notes?
Sadly, it is not possible to change the font used by Preview for Notes - this is denoted by Apple an 'UnEditable'.

This thread on the Apple forums (which I can't take credit for) provides a couple of work arounds such as using Annotations instead of notes (as these are editable) or post-editing comments in Adobe Acrobat Reader XI, but not an actual solution to this problem.