I want the ability to edit PDF files. I prefer not to convert to word.

Editing math-based PDF files is likely to be even more painful. What you have (PDF) is not suitable for your task (editing). Get hold of the original editable file (which may be a Word document, LaTex, InDesign, who knows) and the app for editing it. PDF does NOT bridge the gap of "I can edit anything" much as we all wish we had that Rosetta Stone.