The reason for the issue is the new behavior of the browser described in details in the Compatibility changes in IE11
MSDN article. Currently IE11 represent itself by default with navigator.userAgent
property value Mozilla/5.0.
Generally we read this property to handle per browser the pdf plugin checks and print operation. Thus this new and unexpected IE 11 behavior causes the report viewer to misrecognize the client browser that results in incorrect PDF plugin resolution and the fallback to the export operation.
This issue is already fixed and our recommendation is update your Telerik Reporting installation with the latest official release.
New HTML5/JS REPORT VIEWER with MOBILE AND TOUCH SUPPORT available in Telerik Reporting Q3 2013! Get the new Reporting version from your account or download a trial.