I've been using PDF files since high-school but whether they were for semestrial projects, degree certifications, business proposals, editing their content always proved to be a difficult task. An application that helped me correct and make notes in my PDF files with ease was PDF Annotator.
Although, at first sight, the program might look a bit overwhelming due to numerous toolbar components, you can easily remove the ones you don't need by going to View menu. I tried several similar applications, but none of them offered me so many utilities and geometrical shapes to use while editing my PDF file.
Unfortunately, PDF Annotator also came with a small flaw: in some areas of the loaded document, the program was unable to recognize and extract text contents. More than that, I found pretty disturbing the fact that you can save the resulted file only as a PDF file.
The product seemed to me a bit pricey mostly because I don't get to use PDF Annotator on a daily basis, so it's up to you to decide if it's worth acquiring and trying the application.