There is nothing wrong with your implementation of RadWindow, the problems comes from the HtmlPlaceHolder you use to show the PDF document. We still have one limitation that we are trying to overcome, if you are integrating the HtmlPlaceHolder with RadWindow, you should Update the layout of the HtmlPlaceHolder every time the location of the RadWindow changes. If you open our online example at http://demos.telerik.com/silverlight/#HtmlPlaceholder/WindowIntegration you can notice that the RadWindow calls RadHtmlPlaceholder1.InvalidateArrange(); everytime it changes it's location: the complete callback method looks like this:
void RadWindow1_LocationChanged(object sender, System.Windows.RoutedEventArgs e)
Please get back to us if you have any further problems.
the Telerik team