PDF is really a final form file format for which the ability to edit content is very limited.
You can use the text touch-up/edit function to change the font of text strings, one at a time, assuming (1) the font is installed on your system and can be embedded into the PDF file, (2) all the glyphs used with the original font are in the replacement font, and (3) you are willing to deal with likely changes in line lengths and line wrap. You should also be aware that any special pair or track kerning, ligatures, etc. may be lost since Acrobat's text editing is fairly simple (in other words, forget all those wonderful OpenType features you may have used in your layout program).
Generally speaking, other than for the simplest of text edits, you are better off going to the original source document (InDesign, Illustrator, Word, PowerPoint, etc.) and fixing the issue (in this case changing the font used) there and then subsequently regenerating the PDF file. That is most often the fastest, safest, and most reliable method for significant text edits, especially including changing of fonts.
- Dov