If we had a pound for every time this is asked…. Well, Reffie would be sorted for dino treats for the rest of the year.
PDF stands for Portable Document Format. It is merely a type of file, much like a Word document (.doc) or a Powerpoint (.ppt). It does not in any way represent what the actual document contains: it could be a journal article, it could be a report from a Government website, it could be an extract of an ebook. If you are uncertain about how to reference that material, chances are you are missing some vital bit of information, often the year of publication.
Expect to do a bit more digging! Sometimes, the webpage where the PDF can be downloaded has the missing information you need. Google is your friend for this, but we can help too (firstname.lastname@example.org). It is also best practice to find out who wrote the information in that PDF, how old it is and whether it is intended to be a credible source of information for academic work.
In fact this stands for any source of information – read our Evaluating sources page here!