You cannot send files during a partial postback, so I advise that you move the hidden button out of the RadAjaxPanel:
Also note that you need to use the client-side API of the RadNotification control to show it, i.e. the $find() method, e.g.:
I am attaching here a short video that demonstrates the proper functionality. Note how it does not use AJAX for the postback that will bring the PDF file to the browser.
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