First, take a look at the type of material that the PDF is. Is it an article? What about an image? Most PDF materials are articles as most images are JPEGs or PNG files. Once the "type" is identified, look at determining whether it is a part of a journal or a book or is a stand-alone piece of writing.
Now that the type has been identified, the work should be cited as usual with one exception. At the end of the citation entry, after the last period, add the title "PDF file." This will help readers identify the source and find it if they are interested.
If the PDF cannot be cited as any of the other format, then it must be cited following the digital file format. This format involves listing, in this order, according to Purdue University: "the author's name, the name of the work, the date of creation and the medium of publication." If the medium is unknown list Digital file and then end with the title "PDF file" once again.Learn more about Writing