You can apply page break to the exported PDF using the a specific tag <?hard-pagebreak?> which will be recognized by the export tool. But since this is not a xhtml tag it will be modified by the browser, so to avoid this you will need to use a placeholder tag and replace it on the server before calling RadEditor's ExportToPdf() method.
For your convenience I have attached a sample page demonstrating this approach.
In addition, you can find an example on how to implement a custom button to the RadEditor's toolbar inserting a print page-break. It might be useful implementing page break for PDF:
Implementing a Page-break Button
All the best,
the Telerik team