index.aspx.cs

 // save the document as PDF in a session variable
byte[] data;
TextControl1.SaveText(out data,
    TXTextControl.Web.BinaryStreamType.AdobePDF);

Session["document"] = data;

// create a new IFRAME and call this ASPX with QueryString
// to load document from session data
ScriptManager.RegisterClientScriptBlock(
    this.Page,
    this.Page.GetType(),
    "alert",
    "var iframe = document.createElement('iframe');" +
        "iframe.src = 'index.aspx?download=1';" +
        "iframe.style.display = 'none';" +
        "document.body.appendChild(iframe);",
    true);