RadPdfViewer only loads content which is inside the viewport, which means that you cannot get the UIElements for all pages from the visual tree. An easy workaround would be to put the control inside a StackPanel, which will load all the content and you will be able to retrieve the UIElements for all pages.
Another way you could do this is using the ThumbnailFactory. This will create a snapshot for a given RadFixedPage and you could somehow modify this snapshot according your needs.
I hope this is helpful. Please, do not hesitate to contact us if you have any other questions.
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