I did a tiny bit of research on this issue and for some it is a matter of the email client on the senders side, being too dumb to know what a pdf file is, and be able to send it so renames them to .bin. It can also occur when it renames files with spaces in the file name. So, if you try having them get rid of the spaces, and update their email client, the problem should go away.
If, in webmail, the file is actually a .bin, then that company needs to stop renaming their pdf files to .bin. Also, I’d be worried about dealing with people who cannot attach a file and send it.