you cant convert PDF to a Keynote.
use the Keynote as a presentation itself by using full screen mode in Acrobat reader
if the security settings in the PDF allow, you may be able to copy and paste single pages, images or text into Keynote.
Actually, after more messing around with the file I wanted to convert, I found that if I saved the pdf file as a jpeg using Adobe Acrobat I ended up with a desltoop full of thumbnails which I could then drag into Keynote. Fortunately, the format of the jpeg files were perfect in my particular case.