Hi,
From where you are downloading the Excel file.
if my guess is for example from gridview your exporting data to excel then try like this.
this.GridView1.Page.EnableViewState = false;
StringWriter tw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(tw);
hw.WriteLine("<font size="3"> Test</font>");
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;
var fileInfo = new FileInfo(FilePathName );
fileInfo.IsReadOnly = false;
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();