The PdfViewerControl.UriOpening event allows you to customize the URI opening behavior. The e.Handled property is set to false by default. In this case, the PDF Viewer invokes a Security Warning message box, which warns you that the PDF Viewer will use an external resource to open the URI.
Click Yes to open the URI in the PDF Viewer, otherwise click No. Set the e.Handled property to true if you want to hide the message box.
You can prevent URI opening without using the message box (e.Handled should be set to true) by setting the UriOpeningEventArgs.Cancel property to true.