Thank you for writing.
You can achieve your goal by inheriting from RadPrintPreviewDialog and overriding the OnShowPrintDialog method. This method is called when the user clicks the print button. There you can change the Landscape property of the printed document. Here is the code you can use:
MyPrintPreviewDialog : RadPrintPreviewDialog
To show your dialog you should use the following snippet:
MyPrintPreviewDialog dialog =
RadPrintDocument doc =
doc.AssociatedObject = iPrintableInstance;
dialog.Document = doc;
I hope this will be useful. Should you have further questions, I would be glad to help.
the Telerik team