index.aspx

<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:Button ID="btnStore" runat="server"
            Text="Store document locally" OnClick="btnStore_Click" />
        <input onclick="restoreDocument()" 
            id="btnRecover" type="button" value="Restore" />
        <asp:HiddenField ID="hiddenDocument" runat="server" />
    </ContentTemplate>
</asp:UpdatePanel>