This message is seen directly in my inbox when I just click on the email. I haven't opened the email or clicked on the attachment. (but when I do nothing is different) Since the error message is a one page PDF, I can see it in the body of the email. It is as if the pfd has converted itself to this message only.
Hmm.
If you see an attachment that is an actual pdf file, try saving it without opening it. How you do that exactly varies from one email client to the other but you should have that option. What it sounds like is happening is you are viewing your email in a web browser and your browser is attempting to preview the file in the body of the email but it isn't using Reader. It's using it's own browser. If I had to guess, I would say that this is also supposed to be a form of some type?
In the end, probably not an Adobe Reader issue but more a web browser issue. Try and download that pdf file...