Okular for KDE is a great app. I use it for my studies because it allows you to save your highlighting and annotations in a separate XML file (actually, it does that automatically once you work with a PDF file) and you can easily copy the XML file to another computer and continue working on your PDF there. The only important thing is, that the PDF file name has to be the same.
As the question came up earlier: the XML files can be found in the folder home/.kde/share/apps/okular/docdata (make sure that you can see hidden folders).
As for the PB 903: This is really a shame. I'm a doctoral student and I will probably go for the Entourage Edge... it is heavy and pricy, but once I checked out the presentations on YouTube I instantly fell in love with it. Check this out for annotation options on the Edge:
http://www.youtube.com/watch?v=7bJWCzTpDZ8
Greets