This is a semi-bug in the PDF extension: it adds a special page object during init, thereby triggering MediaWiki's complete init procedure for all special pages. SMW, in contrast, merely registers its special pages for later on-demand loading. Now when using PDFExport, the "later" loading has already happened, without SMW's pages being considered.

A fix, which will also significantly speed up wikis that use PDFExport, is as follows:

• move the class definition for SpecialPdf out of the function wfSpecialPdf()
• remove the line SpecialPage::addPage (new SpecialPdf()); from wfSpecialPdf()
• add the following code to wfSpecialPdf():
  
  global $wgSpecialPages; $wgSpecialPages['PdfPrint'] = array('SpecialPdf');

Thinking about it, the values for $wgSpecialPages should be set up right away during loading, not in the extension hook. I will move SMW's registrations, so that even the current PdfPrint version should not break them. The above change is still useful to improve performance (the impact of addPage can be quite large), but again it should be better to move

$wgSpecialPages['PdfPrint'] = array('SpecialPdf');

right at to top, outside of the function.

Added to the page http://www.mediawiki.org/wiki/Extension:Pdf_Export