If I understand you correctly, the issue may be related to your email program more than anything to do with Adobe Reader. No paperclip in the message itself? If the pdf file showed up, you should be able to save it somehow to your hard drive. If that isn't visible to do, then there's something else going on.

Then right click on the file name and choose which program to use. On Windows 10, Edge should work, but I've never had trouble with Reader and have that as my default.

What email program are you using?