The PdfViewer DocumentSource provides a read-only property IsBusy which indicates when the control is loaded. Basically, when the IsBusy changes its state to false the document is completely loaded and the FitToWidth command can be executed.
Currently, the PdfViewer does not provide a DocumentLoadedEvent. We have this logged as a feature request in our Feedback portal and you could vote for the item on the provided link below:
Please take a look at the attached project how the FitToWidth command is executed when IsBusy is false.
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