»PdfExport Ausgabe 0kb groß und kann nicht geöffnet werden

You'll need to make sure the paths are right(put htmldoc in Path environment variable). If you're having trouble with the images rendering, you may have to fiddle with how the pathnames get generated for them. In general the easy way to debug this extension is to comment out the line that removes the temp file. That way you can both check and hand-code the temp file, as well as manually run it through htmldoc to see what happens. Try commenting out $bhtml = str_replace ('/w/',$wgServer . '/w/', $bhtml);