Poorly. PDF can be readable or image. If image, you have to convert it to characters using an OCR. If readable, it is minimally formatted. .pptx is a presentation slide format. Best way is to manually use copy paste and pretty up the results.
Now when someone is looking for the PDF file (and they can see your deck and thumbnails, no need for having to search for your deck and thumbnails, it's easy to put them in at the touch of a button!) And when you search Google on your deck name it should show up as Google Docs (or whatever) PDF without any problem.