This article lists some common problems and difficulties related to pasting in RadEditor using Internet Explorer.
The most common reason for this problem is that the client-side programmatic access to the clipboard is disabled.
Enable the programmatic clipboard access so that the built-in mechanism of RadEditor can properly process the content to be pasted. This MSDN article shows how you can enable this option: https://msdn.microsoft.com/en-us/library/bb250473%28v=vs.85%29.aspx.
With IE11, pasting from MS Word is a native feature. Before RadEditor can processes the content, IE11 has already transformed the MS Word content to HTML. Thus, the built-in mechanisms of RadEditor can only receive that plain HTML content that is being pasted and does not improve it.
You can read more about how RadEditor handles pasting in the Pasting Content Overview article.