The RadWindow loads the page in an iframe and thus behaves like one. If you point the NavigateUrl property directly to a PDF the document will be loaded in the RadWindow (or the user will be prompted to save it, depending on the browser settings). If you are streaming a file from an aspx page please make sure that you set the correct headers so that the browser will know what is coming, instead of trying to interpret it as text/html:
a list with the available properties
one mime type reference
one discussion on the types
I would advise that you start by making it work propertly in a regular iframe in IE, too, then the same code should work for the RadWindow as well.
the Telerik team