Well if you want to find a perfect XHTML+CSS to PDF converter library, forget it. It's far from possible. Because it's just like finding a perfect browser (XHTML+CSS rendering engine). Do we have one? IE or FF?
I have had some success with DOMPDF. The thing is that you have to modify your HTML+CSS code to go with the way the library is meant to work. Other than that, I have pretty good results.
See below:
Original HTML
Converting HTML to PDF
