Your pdf needs to be in a .zip archive. The .zip is what you'll upload. For a PDF, you'll want 'Launch with' to be set to 'DEFAULT APPLICATION'.
You might want to test if your PDF opens with mupdf as users with older images are likely to have this as their default PDF reader; Xpdf should be on the newest Raspbian image. If your PDF doesn't open in mupdf (as many haven't), you might want to leave a note in the project description reading along the lines of "Requires Xpdf to open" to make this clear, or you could make an install script which installs Xpdf and creates the file association for it.
The upload process is documented here
Please don't hesitate to ask if you have any other questions.
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