RadPdfViewer is an UI control and if you want to show a PDF document in it, you need to make sure the control is in the visual tree. There are several approaches you could adopt in this regard, one of them being to dynamically create the control, assign a document to it and add it to the root of the window. There are plenty of discussions on the topic in internet, for example in this StackOverfow thread
and here is a simple example.
If your window contains a Grid definition and you want to place the RadPdfViewer in the first row of the grid, give the Grid control a name in the XAML.
Then, when you want to show the pdf viewer just create the control, assign a document to it and add it as a child of that grid.
RadPdfViewer pdfViewer =
An alternative approach is to create a separate window in your application that contains a RadPdfViewer. Every time you need to show a PDF document just instantiate the window and call its Show() method. There is an example of that approach in this MSDN tutorial
I hope this helps.
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