Failed PDF send aborts further actions
If you create a CiviRule that sends an email, and the contact doesn't have a valid email, the email is skipped, processing continues.
If you create a CiviRule that sends a PDF, and the contact doesn't have a valid email, an exception is thrown and processing stops.
Commenting out the line return civicrm_api3_create_error(implode("\r\n", $pdf->getErrors()));
solves the problem, but I'm not sure what the correct solution is. It seems like the two extensions should be consistent though.