My problem is how to download a RTF on the browser using a button inside an UpdatePanel, in my case a button inside a GridView:

Insert in the update panel a hidden iframe with runat=server

<atlas:UpdatePanel ID="UpdatePanel1" runat="server" >
<ContentTemplate>
<iframe id="frameGenerateRTF" runat=server src="about:blank" width=0 height=0></iframe>
.... GridView code.....      

On the GridView1_RowCommand that generates the RTF export change the src of the IFRAME pointing to the page that generates the binary export

frameGenerateRTF.Attributes.Add("src","Export.aspx?ACTION=PRINT&IDDOCUMENT=12345");

The Export.aspx page uses the Response.BinaryWrite and Response.End() to write a stream (PDF,RTF etc..) to the browser.

You can't use the Response.Redirect on the same page because when you write the binary stream you interrupt the processing on the page usig Response.End() and the atlas page end to respond.