No, the problem isn't with a Browser setting. I tried this using IE7, IE8, FF, Opera, and Chrome. All the same result.
I'm not using a Windows server. My Moodle is hosted on a Unix server with PHP version 5.2.1 with Apache.
You can see the "view" file name. I encourage you (also) to go to Your Ceretificate, save Your Certificate to your desktop. Right click the file, select properties. The file properties will show you the complete file name.
In Microsoft Windows, by default all common file extension names are hidden. Thus you will see "view" as opposed to "view.php" The "PHP" is called the FILE EXTENSION. This is where the problem lies. THE FILE EXTENSION NAME in which Moodle saves the certificate.
After saving the certificate to you desktop, for example, I cant open it with Adobe reader because of the file extension name. The extension name needs to be renamed to PDF. Then, and only then, will Adobe Reader open the certificate.