<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"/>

private void DocumentChanged(object sender, DocumentChangedEventArgs e)
        {
            pdfViewer.ScaleFactor = 1.3;
            var size = pdfViewer.Document.Pages[0].Size;
            pdfViewer.ScrollToVerticalOffset(size.Height * 0.3);
            pdfViewer.ScrollToHorizontalOffset(size.Width * 0.20);
        }