By design, the parameters area for the HTML5 based viewers is positioned on the right-hand side. To change its location, you can customize the viewer's HTML template or apply custom CSS to the viewer's elements. Modified HTML template will have to be linked on the page explicitly by specifying the viewer's templateUrl option. The HTML5 Viewer's default HTML template files are located in Telerik Reporting installation folder, for example C:\Program Files (x86)\Progress\Telerik Reporting <VERSION>\ReportViewer\templates.
For additional information, check Customizing the HTML5 Report Viewer help articles.
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