Add your code in button click add this code (replace your link in the code)
private void btn_Click(object sender, EventArgs e)
{
.....
StringBuilder script = new StringBuilder();
script.Append("<script>");
script.AppendFormat("window.open('{0}', '', '');", link);
script.Append("</scri");
script.Append("pt>");
Page.RegisterStartupScript("opennewwindow", script.ToString());
.....
}