Use
showModalDialog
like as below
RptDoc.ExportToDisk(ExportFormatType.PortableDocFormat,Server.MapPath("/SRVRpt.pdf"));
string popupscript = "<script language="'javascript'">" + "window.showModalDialog('/SRVRpt.pdf','','');</script>";
Page.ClientScript.RegisterStartupScript(this.GetType(),"PopUpWindow", popupscript, false);