Making exported excel sheet as readonly

In my application i'm exporting gridview data to excel and storing it to a particular folder now what i want is to make this excel file as read only so that no one should edit it.

i have written code like this:

protected void Button5_Click(object sender, EventArgs e) {

        this.GridView1.Page.EnableViewState = false; 
        StringWriter tw = new StringWriter(); 
        HtmlTextWriter hw = new HtmlTextWriter(tw); 
        hw.WriteLine(" Report"); 
        this.GridView1.RenderControl(hw); 
        string HtmlInfo = tw.ToString(); 
        string DocFileName = "Report" + ".xls"; 
        string FilePathName = Request.PhysicalPath; 
        FilePathName = FilePathName.Substring(0, FilePathName.LastIndexOf("\\")); 
        FilePathName = @"C:\Excel" + "\\" + DocFileName; 
        FileStream Fs = new FileStream(FilePathName, FileMode.Create); 
        BinaryWriter BWriter = new BinaryWriter(Fs,System.Text.Encoding.GetEncoding("UTF-8")); 
        BWriter.Write(HtmlInfo); 
        BWriter.Close(); 
        Fs.Close(); 
 
   } 

any1 help me on this...