This issue is commonly associated with an SMTP plugin conflict. To test, disable the SMTP plugin and test the form.
If you are not using an SMTP plugin, then create a test form and test PDF template (not the same PDF) to see if the issue is limited to one form or a sitewide issue.
If the test form works, the issue may have to do with the PDF document, please send a copy to support AT formidablepro2pdf.com
If the issue is sitewide, we would need to log in and troubleshoot the issue, please use this form to provide credentials.
We remain at your service.