Their own documentation tells you exactly how you can export to a PDF file:
http://www.mediawiki.org/wiki/Extension:Pdf_Export
http://www.mediawiki.org/wiki/Extension:PdfBook
Or you can export the files in a way another developer can upload to their own mediawiki:
http://www.mediawiki.org/wiki/Manual:Parameters_to_Special:Export
Or dump all the files out as HTML that you could zip up and transfer: