C# – How to execute multiple ClientScript.RegisterStartupScript in ASP.NET c#

I'm developing a gridview in which you can download multiple files with one button.

Here's my gridview:

<asp:GridView ID="grdvHistorialMensajes" runat="server" AllowPaging="True" 
                    AutoGenerateColumns="False" CellPadding="4" AllowSorting="true"
                    EmptyDataText="No Hay Mensajes Enviados" ForeColor="#333333" 
                    GridLines="None" CellSpacing="1" 
                    onpageindexchanging="grdvHistorialMensajes_PageIndexChanging" 
                    onrowcommand="grdvHistorialMensajes_RowCommand" 
                    onsorting="grdvHistorialMensajes_Sorting">
                    <AlternatingRowStyle BackColor="White" ForeColor="#284775" />

                    <Columns>
                        <asp:BoundField DataField="CorreoCliente" HeaderText="Correo Del Cliente" SortExpression="CorreoCliente" />
                        <asp:BoundField DataField="CorreosAdicionales" HeaderText="Correos Adicionales" SortExpression="CorreosAdicionales" />
                        <asp:BoundField DataField="Tema" HeaderText="Tema" SortExpression="Tema" />
                        <asp:BoundField DataField="Mensaje" HeaderText="Mensaje" SortExpression="Mensaje" />

                        <asp:TemplateField HeaderText="Fecha" SortExpression="Fecha">
                            <ItemTemplate>
                                <%# DataBinder.Eval(Container.DataItem, "Fecha", "{0:dd/MM/yyyy}")%>
                            </ItemTemplate>

                            <EditItemTemplate>
                                <asp:TextBox ID="tbxFecha" runat="server" Text='<%#Bind("Fecha","{0:dd/MM/yyyy}") %>' ValidationGroup="gpEdicionAgenda">
                                </asp:TextBox>
                            </EditItemTemplate>
                        </asp:TemplateField>

                        <asp:BoundField DataField="Hora" HeaderText="Hora" SortExpression="Hora" />
                        <asp:BoundField DataField="Archivos" HeaderText="Archivos" SortExpression="Archivos" />

                        <asp:TemplateField>
                            <ItemTemplate>
                                <asp:ImageButton ID="imgBtnDescargarArchivos" runat="server" 
                                    CommandArgument='<%# Eval("IdMensaje")%>' CommandName="Descargar" Height="16px" 
                                    ImageUrl="~/img/activar.png" ToolTip="Descargar" Width="16px" />
                            </ItemTemplate>
                        </asp:TemplateField>

                        <asp:TemplateField>
                            <ItemTemplate>
                                <asp:ImageButton ID="imgBtnVerMas" runat="server" 
                                    CommandArgument='<%# Eval("IdMensaje")%>' CommandName="VerMas" Height="16px" 
                                    ImageUrl="~/img/search.png" ToolTip="Ver Mas" Width="16px" />
                            </ItemTemplate>
                        </asp:TemplateField>
                    </Columns>

                    <EditRowStyle BackColor="#999999" />
                    <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                    <HeaderStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                    <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
                    <RowStyle BackColor="#F7F6F3" ForeColor="#333333" HorizontalAlign="Center" />
                    <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
                    <SortedAscendingCellStyle BackColor="#E9E7E2" />
                    <SortedAscendingHeaderStyle BackColor="#506C8C" />
                    <SortedDescendingCellStyle BackColor="#FFFDF8" />
                    <SortedDescendingHeaderStyle BackColor="#6F8DAE" />
                </asp:GridView>

Whenever I click on the "Descargar" RowCommand, I originally used this:

if (e.CommandName == "Descargar")
            {
                DataTable dt = ConexionBD.GetInstanciaConexionBD().GetArchivosPorMensaje(Convert.ToInt32(e.CommandArgument));

                foreach (DataRow dr in dt.Rows)
                {
                    string strArchivo = dr["Nombre"].ToString();
                    string strExtension = Path.GetExtension(strArchivo).ToLower();
                    Response.Write("<script>window.open('/Archivos/" + strArchivo + "');</script>");
                }
            }

When I clicked, if that row had let's say 1 pdf, 1 jpg and 1 doc, it opened both the pdf and the jpg in a different window and the doc would be downloaded. That's exactly what I want. However, I noticed that whenever a new page is opened (in the case of the pdf and jpg) all the font in the page is altered. So I wanted to find a solution and then I tried this:

if (e.CommandName == "Descargar")
            {
                DataTable dt = ConexionBD.GetInstanciaConexionBD().GetArchivosPorMensaje(Convert.ToInt32(e.CommandArgument));

                foreach (DataRow dr in dt.Rows)
                {
                    string strArchivo = dr["Nombre"].ToString();
                    string strExtension = Path.GetExtension(strArchivo).ToLower();
                    ClientScript.RegisterStartupScript(this.GetType(), "myFileOpenScript", "<script>window.open('/Archivos/" + strArchivo + "');</script>");
                }
            }

When I open a pdf file, the font is not altered this time, however, It would only open/download the first file that appears int dt.Rows[0] (dt.Rows[1] on won't open). I suppose that a Response.Write can be deployed multiple times, however, a ClientScript.RegisterStartupScript probably can only be executed once.

Is there another method I can use to not alter the page's letter font and to open multiple files with a single click?

Or how could I execute ClientScript.RegisterStartupScript multiple times??

Thanks In Advance