The described behavior is encountered due to incompatibilities of the RTF document opened in MS Word 2013.
When MS Word is opened in compatibility mode
(when RTF document is being opened) the word breaking feature does not work correctly.
Using the built-in export of the RadEditor control cannot somehow manipulate this functionality. This entirely depends on the desktop word processor and the exporting functionality cannot configure this matter.
For such cases I would recommend exporting to DOCX document, but this is still not available as a built-in option in the RadEditor control.
A possible way to allow users to export to DOCX is to upgrade to 2014 Q2 version with which the Telerik UI for ASP.NET AJAX comes with the Document Processing Library with which a custom exporting functionality can be implemented. More about this matter is available in this help article
and this Code Library
that provides a simple project with the Document Processing Library integrated with the RadEditor control.
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