This problem occurs when PDFViewCtrl.Dispose is explicitly called by the user code. GC will try to collect the PDFViewCtrl whether or not PDFViewCtrl.Dispose has been called explicitly. GC will then throw this exception when it tries to dispose an already disposed PDFViewCtrl. This is a feature of the Visual Tree, out of our control. GC generally kicks in when you create a new PDFViewCtrl and set a document.
In order to ensure that the PDFViewCtrl is reclaimed properly, ensure that every event handler subscribed to the PDFViewCtrl is unsubscribed when the PDFViewCtrl is no longer needed. Otherwise, an island will be left that the GC will not be able to collect.