Hi,
I have had a similar problem. The loading icon was spinning forever and nothing happend for the user. But the user and me received the mail anyway.
In the top of the “Send PDF for Contact Form 7 – Settings” page, when i selected my formular there where a lot of warnings like
‘A non-numeric value encountered in /var/www/html/wp-content/plugins/send-pdf-for-contact-form-7/mpdf/Tag.php on line 1889’
I solved this problem by removing all html list (in my case just unordered list) elements and after this warning messages disappeared. And my formular was working fine again (loading icon finished, done message was displayed and mail was sent including the pdf).
