Note that Report Viewer is part of Telerik Reporting and you can use it to export the data to Excel, Word and PDF. If you intend to use it you can chack out the following help article
which elaborate more on passing grid's data to Telerik Reporting. After you get the data you need to pass it to the Report Viewer that needs to be create as described in the following articles
Otherwise if you want to use the built-in export functionality of RadGrid you can use the firstly provided example.
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