Acrobat is not a word processor or a desktop publishing system). so it's support for things that are easy to do in e.g. MS Word are not necessarily things that Acrobat is good at. It's always best to make these kinds of changes in the original application, and then convert to PDF. As an alternative, you can try to export the PDF as a Word document, make the necessary changes in Word, and then convert back to PDF. I am not aware of a method in Acrobat to do a find and replace all.