When emailing an invoice, the PDF file that is attached to the email has a random four digit number at the end of the filename.
This occurs when multiple instances of QuickBooks are open with different files and an email is sent from one of the QuickBooks instances. The number at the end of the file name is the process ID of the QuickBooks instance that created the file.
To email the PDF without the number at the end of the file name, close out of one instance of QuickBooks and send the invoice. After it has been sent, reopen the second QuickBooks instance and continue your work.