According to the code in the Github Repo, ProcessDestination calculates the client width and height in order to process FitR, FitBH, FitBV destinations. In doing so it is subtracting off the system scrollbar width and height from the ActualWidth and ActualHeight properties of the viewer. The trouble is that in WPF, the pdfViewer needs to be put into a ScrollViewer in order to even have scrollbars and the ScrollViewer can have a control template that does not use the system scrollbar sizes. I think the ActualWidth and ActualHeight by itself is the proper value to use because it doesn't include the scrollbar width and height.