For simple XML, PDF generation is built into Splunk 5.X (and I must admit : is not bug-free...)
Simple XML with forms
Advanced XML with forms
You have to use the PDF report server. Installing this app is not very trivial but since I've done it, I've noticed there is a new release. Most of the problems I remember were linked to undocumented missing dependancies, but basically, if you install "firefox" on your PDF server, you should pull out all the necessary packages and libraries.