Raphael,
You can’t convert directly from XML to PDF with XSLT. What you can do is convert from XML to an XSL-FO (Formatting Objects) document using XSLT. You then take the XSL-FO document and run it through a rendering engine to produce your PDF.
A widely accepted, freeware (I believe) rendering engine is FOP. More advanced functionality is also available via commercial packages from the likes of RenderX. Sorry I don’t have links to them handy, but they should be easy enough to find on the Web.
I don’t have much hands-on experience doing this, but if you like I can point you to some better resources on the subject. HTH.