If the document was not created in Illustrator, then trying to edit the text in Illustrator could be messy and problematic. Sometimes what I resort to is placing the entire PDF in a new InDesign document, drawing a white box over the text I want to replace, and putting new text over the top of that. It's not an elegant solution, but it works.
You could also open the PDF in Photoshop and rasterize it as a 300 ppi grayscale image, erase the offending text and put new text in place. That's even less elegant, but again, it works for the short term.
