For PDF files, I would recommend using
Mobipocket Creator to convert them to Mobipocket files. This creates very readable eBooks for most PDF files. If the PDF has more than 1 column or tables then the result is less good. If your PDF files are primarily text with a few pictures this conversion should work well for you.
You can open a PDF file on the Cybook without converting it but the fonts will be very small as most are created with a letter size page in mind. The Cybook has a good 'zoom' feature which makes the PDF readable but you may have to scroll sideways as well as have multiple screen 'pages' per PDF page. There are tools available which can crop the blank margins and give you a smaller page size which fits better on the small screen but I have not been motivated to try them out.