Free Editor Able to Edit PDF Forms

Guys, a friend of mine is a tutor teaching budding electricians here in NZ.  He will have to mark their exams which he tells me are PDF forms with fields for right or wrong and a box for comments.  He is not flash with PDF and would like a free editor to do the job with.  Can you advise?  He tells me he has a decent Internet connection so an online app will work for him.

Note: I have not Googled yet but will do so, but ISTM that this forum is going to have a way better idea than me what to use.

[EDIT]  I should have mentioned that he uses Windows.

TIA