"This site can't be reached" error message when trying to

This error is unfortunately not enough information to go on. I would suggest checking your CiviCRM log and perhaps your web server's error log as well.

If I had to hazard a guess though - it's possible that some element of your Joomla theme is adding text (perhaps a header or footer) to every page - and this is causing the PDF to be incorrectly formatted. If that's the case, you should see this same error when you print any PDF.