The CurrentPageTextBox in the toolbar has stopped displaying in the latest version.
Workaround:
<telerik:CurrentPageTextBox x:Name="PART_CurrentPageNumberTextBox"
HorizontalContentAlignment="Center"
Margin="2" Text="{Binding FixedDocumentViewer.CurrentPageNumber, Mode=TwoWay}"
VerticalContentAlignment="Center" Width="45"
telerik:StyleManager.Theme="Office_Black"/>