System.Diagnostics.Process p = new System.Diagnostics.Process();
    p = new System.Diagnostics.Process();

    p.StartInfo.FileName = CreatePDF(); // method that creats my pdf and returns the full path

    try
    {
        if (!p.Start())
            Controller.Error = "Opening acrobat failed..";
    }
    catch(Exception ex)
    {
        Controller.Error = "Create PDF::" + ex.Message;
    }

Response.ContentType = "application/pdf";
Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", file.FileName));
Response.BinaryWrite(file.FileBytes);
Response.Flush();
Response.End();

var stream = GetTheFileAsStream();
var attachment = new Attachment(stream);

using (Process p = new Process())
{
    p.StartInfo.RedirectStandardOutput = false;
    p.StartInfo.FileName = @"C:foo.pdf";
    p.StartInfo.UseShellExecute = true;
    p.Start();
    p.WaitForExit();
}

public ActionResult GetFile()
{
    return File("foo.pdf", "application/pdf");
}