The InsertPageBreak methods of both RadDocument and RadRichTextBox add the line break at the current caret position and the default caret position is at the beginning of the document.
You can insert the line break at the end of the document by moving the caret position to the end of the document like this:
More information on document positions can be found here
Note that if you are not showing the document in a RadRichTextBox in the visual tree, you need to measure and arrange the document before being able to use methods from RadDocument's API:
document.Arrange(new RectangleF(PointF.Empty, document.DesiredSize));
I hope this helps.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items