ASP.NET Web Forms Report Viewer
This is a legacy report viewer and for new projects our recommendation is to use the latest web forms report viewer - HTML5 Web Forms Report Viewer
The ASP.NET ReportViewer is designed to render Telerik Reports within ASP.NET WebForms projects.

The ASP.NET Report Viewer uses ASP.NET Session and ASP.NET Cache in order to preserve the report instance assigned to its Report property during page postbacks and for some design reasons as well, so Session should be enabled at all times!
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Export: see Rendering Extensions for available export formats.
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Print Preview: switches between Logical and Physical page renderer. See Rendering and Paging for more info.
Print: see True Print for more info.
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How it works
The ASP.NET ReportViewer control is a composite of standard ASP.NET Web Server controls.
The ASP.NET ReportViewer comes with four predefined skins: Default, WebBlue, Original and Office2007. The ASP.NET ReportViewer control's toolbar is immutable. To build custom UI for the toolbar and work with the rendered report in the viewer, you can use the ASP.NET ReportViewer Client-Side API.
The purpose of the ASP.NET ReportViewer control is to display Telerik Reports and allow the user to interact with them. Reports are processed and rendered on the server. Reports and their content are delivered to the client through the Telerik.ReportViewer.axd handler.
The ASP.NET ReportViewer control's content is rendered in IFRAME elements. The report in the viewer is rendered through the IMAGE rendering mechanism based on the Design Considerations for HTML Rendering.