A set of technologies in the .NET Framework for building web applications and XML web services.
Most likely it is not a problem in your code but in IE. Contrary to modern browsers, IE does not have a built-in PDF viewer, you need to install it as a plug-in from third parties. For instance, if I recall correctly when you install Acrobat Reader it adds a plugin for IE.
If the plugin is not installed, then IE treats it as an "unknown file type" and just downloads it without displaying.