When I wrote my article Zoom and page chaged events for FlowDocumentPageViewer I make mistake, you can read about it - there. However Martin offer solution for this problem. But I don't like this solution, overriding of OnPropertyChanged isn't best solution because this method is risen in many cases so any logic there is serious performance issue.
So how can we fix it in other way? We can find object of Slider in Visual Tree of FlowDocumentPageViewer and subscribe to ValueChanged event and call OnZoomChanged(); there.
Bingo! It fixes my trouble.