When the user clicks an thumbnail, the associated PagesViewer control will navigate to the corresponding page. Also, the ThumbnailsViewer will indicate with a red border what pages are currently visible in the DocumentViewer control.

Please note that the ThumbnailsViewer shares its RenderSettings with the associated PagesViewer, so any changes to the RenderSettings of the ThumbnailsViewer also have effect for the PagesViewer and vice versa.