Use the ClientInstanceName property to specify a unique client-side identifier for a Report Viewer control. The ClientInstanceName property's value can be used on the client side to programmatically access the client object rendered for the Report Viewer in client-side script. This property is particularly important in referencing the Report Viewer when it is contained within a naming container (for instance, within an ASPxPageControl's page or an ASPxPopupControl's window).
If the ClientInstanceName property is not specified, a Report Viewer's client identifier is generated automatically and equals the value of the Report Viewer's ID property. Note that in this case, client-side programmatic access to the Report Viewer by its ID is not allowed when the Report Viewer is contained within a naming container.