The extension on the downloaded file (as shown in the attachemnt) is .zip, NOT .pdf
Nope. HTTP headers don't lie, random windoze installs do.
Relevant parts of http header when downloading that attachement:
HTTP/1.1 200 OK
...
Content-Type: application/octet-stream
...
Content-Disposition: attachment; filename*=UTF-8''DSA815%2010Hz%20Clock%20Frequency%20Adjustment%20.pdf
...
Mmmh, seeing that header, it could also be you are using internet explorer XYZ or something. Mickeysoft is famous for having their own ideas on how to interpret standards. Anyways, I would expect
right-click: save as ... whatever.pdf to work properly in such a case.