Hi there,
At the moment, the only fully working wiki-to-something converter is http://tools.wikimedia.de/~magnus/wiki2xml/w2x.php (this page converts a page of wikipedia to some output format, including openoffice, which can later be exported as a pdf). I suppose it is possible to get the source code.
Actually, you can just use "Print" functionality of your browser to get some PDF ('print to file' under Linux and then ps2pdf to get PDF from PS or something like PDFCreator under Windows). (Theoretically, it is possible to use Firefox or Konqueror on the server side to generate PS/PDF automatically, but it is not very elegant solution.)
You can also try some XHTML+CSS-to-PDF converters, like http://www.digitaljunkies.ca/dompdf/, but there's no free fully CSS-compliant converters AFAIK (if you know one, let me know, please).