I want to add a link through where customer can download file.How can i do that.Please help.

You can have a look at the following articles:

1- Downloading Files in ASP.NET

2- Build Smarter ASP.NET File Downloading Into Your Web Applications

3- Downloading a File with a Save As Dialog in ASP.NET

Hope this helps,
Regards

 in the links click event you can do

Response.Redirect("locationofthefile.filetype");

Just send the full file name of the file that as to be downloaded.

 private void DownloadFile(string FileName)
 {
        Response.AddHeader("Content-Disposition", "attachment;filename=" +              HttpUtility.UrlEncode(FileName));

        Response.ContentType = "application/octet-stream";
        Response.TransmitFile(FileName);
}