Sending out PDF invoices and the documents are received but they are corrupt and can not be opened. Any idea why the email is corrupting PDF's?
Without a lot of work it's impossible to say, but I will say that if you are needing to send PDF invoices in a secure manner then it's very hard to beat Adobe Acrobat Pro (DC). This is very valuable if sending quite a few invoices a month, as signatures etc can be digitally signed, as well as a record kept by Adobe for future reference.
Or you could use the open software 'pdftk' for use at the command line:
But I wouldn't send by non secure email.............