Download feature not working within update panel in asp.net

To initiate a full page postback, you add a postback trigger to your update panel:

<asp:UpdatePanel runat="server">
    <Triggers>
        <asp:PostBackTrigger ControlID="YourControlID" />
    </Triggers>
    <ContentTemplate>
        .....

You cannot return an attachment in an UpdatePanel partial postback, since the results are used by the ScriptManager to update a DIV (not the whole response). The simplest fix for what you're trying to do would be to make your download button as a postback control. That would cause that button to initiate a full postback. Here's the code below to include in your Page_Load

ScriptManager.GetCurrent(this.Page).RegisterPostBackControl(this.lnkDownload);