What a toolbar shows about a PdfViewer: the current page, the page count, the zoom and whether a document is loading.
public PdfViewerState(int PageIndex, int PageCount, double Zoom, bool IsLoading)
The zero-based page at the centre of the viewport (CurrentPage).
The number of pages, 0 while nothing is open.
The zoom as a multiple of actual size (1 = one point of the page is one point on the screen).
Whether a document is being opened.
Spec #33 track 6 shell (issue #221).