I’ve had same bug. It’s related to cookie generated by this plugin and luckily it’s fixable as DIY.
Go to Plugins > Plugin editor > Select plugin to edit: Send PDF for Contact Form 7 > Select. Than navigate to file classes > send-pdf.php and look for this line:
if( empty($_COOKIE['pdf_uniqueid']) ) { wp_redirect( admin_url('admin.php?page=wpcf7-send-pdf') ); exit; }
If you comment it out (either with // at the beginning or /* .. */) it will work as intended, without redirects.
