If you've configured customers to receive PDF visit reports by e-mail, there is a way to check if they have actually received them.
From the Visit Report page
Click the arrow next to the PDF Report button
Click Email Visit Report... option
The information panel that pops up shows you a list of all e-mails sent for that visit report.
It shows those sent to customers and also copies sent to internal staff too.
The status column confirms when the e-mail was sent and recieved and if there was a problem.
You can use the action button at the end of e-mail to resend a copy
There is also an option at the bottom of the info page to send the PDF report to a new e-mail address too.