If you're using Tiger or Panther, you can simply open the PowerPoint document in Microsoft PowerPoint, then press "Print" and print it to a PDF via the built-in PDF options in the print dialog. I believe you can do the same with NeoOfficeJ as well.
Short answer: you cannot directly convert PPT to PDF. You must be able to open and view the PPT file first (in Microsoft Office or NeoOffice, etc.), then printing it to a PDF is trivial.