A great first effort! Many thanks!
After some use I do have these suggestions:

1. When User changes LayoutMode the ZoomLevel property changed event should also be fired. I have a Numeric Textbox to adjust viewer ZoomLevel and would like to see its text update on a LayoutMode change. However when LayoutModeChanged fires the ZoomLevel has not yet changed nor does ZoomLevelChanged later fire.
2. The toolbar item for Fit to Width should be a Toggle in all cases, i.e. toggling between Fit Width and Fit Height (or Fit Page) and should be effective at all times (sometimes has no effect if LayoutMode changed to Single Page).
3. The toolbar item to toggle LayoutMode should honor an assignment of its text property.
4. I'm currently using the change in VisiblePagesCount to determine the document is visible. However a "Loaded" event might be even better.
5. I would like to release the viewer's memory of the document before closing the XAML page. An Unload command would be helpful. For now I just assign Source = null.