<asp:Button runat="server" ID="btn1" OnClick="btn1_Click" Text="DownLoad Multiple Files" />
    <asp:Repeater runat="server" ID="rptr">
        <ItemTemplate>
            <iframe src="<%#Container.DataItem%>"></iframe>
        </ItemTemplate>
    </asp:Repeater>

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Collections.Generic;
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btn1_Click(object sender, EventArgs e)
    {
        List<string> files = new List<string>();
        files.Add("data1.zip");
        files.Add("data2.zip");
        files.Add("data3.zip");
        files.Add("data4.zip");
        rptr.DataSource = files;
        rptr.DataBind();
    }