C#.NET Problem with locking of page after writing stream to window

The code below works fine, the problem is the 1st two lines are appearing on screen. It basically loads the PDF but I can't see the btnAcceptProposal change name or become enabled. Tried a Thread for the load but it broke. Anyone got any ideas - i开发者_如何学Ct's a web application

protected void btnPDFProposal_Click(object sender, EventArgs e)
{    
        btnAcceptProposal.Enabled = true;
        btnAcceptProposal.Text = "sss";

        byte[] p = Order.Proposal;
        Response.AppendHeader("content-disposition", "attachment; filename=Proposal.pdf");

        Response.ContentType = "Application/pdf";
        Response.Clear(); 

        Response.BinaryWrite(p);
        Response.End();
}

You are simply squirting out a PDF file in your responserather than doing a normal postback so where/how/when are you expecting btnAcceptProposal to change?