PDF has a very limited editing capacity, even with Acrobat Pro, mainly geared towards fine-tuning it during pre-press work. Editing is complex, unreliable and basically the best way to do it, is BEFORE the PDF is created, which is not much help, when there's no access to original file, obviously.
To actually edit the text, you need to have that particular font on your system, embedded subset not being restricted (or full font embedded), etc, etc. Formatting most likely will go haywire, again depending on how the PDF was created. To preserve the formatting, and if you only want to remove a certain part of the text, best to select the 'no colour' option in properties - that will keep the document structure intact.
Adding text, on the other hand, is very easy, it's a function called Typewriter in Acrobat, and there are similar ones in other PDF apps, starting with Preview, of course (Show Edit Toolbar --> Text Tools).