The PDF files exported from RadRichTextBox comply with the latest version (1.7) of the standard and can be opened by the most commonly used PDF viewers which support this version. We have noticed that when Adobe Reader opens an exported document, it does some modifications on the file which is the reason for the saving prompt. When another program, for example Foxit Reader, opens a .pdf file exported from RadRichTextBox, it doesn't do similar modifications.
On a side note, as far as we have researched the issue, the changes that Adobe Reader performs on the document are not due to incorrect content of the document. Nonetheless, we have added this issue to our to-do list and will further investigate the cause of this behavior. The task hasn't been scheduled yet though because it does not look like a bug in the export and is not critical.
the Telerik team