Did the plugin ever work on your WP installation?
If yes, what changed since it worked.
If no, try disabling all other plugins except Formidable Forms and then activating PRO2PDF. If the plugin activates, then reactivate the other plugins one-at-a-time until an error is found. Then, let us know the result.
Complete this form if you would like us to log in and troubleshoot the issue.
We remain at your service.