When we email invoices / credits to customers, every so often we get a response that "I can't open the file you sent me!" As far as we know, the files are a standard PDF format file. Does anyone else have the same experience, and if so, do you have any idea why this happens?

Many times, I think this is on someone's phone. I am trying to find out if it is an I-phone or Android. I saw one persons phone where the attachment showed up as a "windows.dat" file! What is that?? How does the PDF get converted to a .dat file?

Is this problem something that we need to solve as the sender of the file, or is this a problem on the receiving end of the file ... our customer's phone or computer issue.

I just got another call ... this one from a Windows 10 user, said there was "no attachment", yet if I look in the send mail folder, there is a message that shows a PDF was attached.

This is so confusing, and irritating for some of our customers. I would really like to find out how to resolve this issue.