I found a bug in PdfExport: REL1_30 (2017-09-21T22:16:05) where the pages only export to pdf if the page name is one word. For example:
/MediaWiki/index.php?title=Special:Pdfprint&page=Page
exports fine, but
/MediaWiki/index.php?title=Special:Pdfprint&page=Main_Page
exports an almost empty pdf.
The fix was to merely replace the underscores in the URL with spaces. To do this, I changed the following lines in PdfExport.php (in the extensions folder) from this:
$nav_urls['pdfprint'] = array( 'text' => $skintemplate->msg('pdf_print_link')->text(), 'href' => $skintemplate->makeSpecialUrl('PdfPrint', "page=".wfUrlencode("{$skintemplate->thispage}"))
to this:
$page_name=wfUrlencode("{$skintemplate->thispage}"); $page_name_with_spaces=str_replace('_','%20',$page_name); $nav_urls['pdfprint'] = array( 'text' => $skintemplate->msg('pdf_print_link')->text(), 'href' => $skintemplate->makeSpecialUrl('PdfPrint', "page=".$page_name_with_spaces)
then run update.php