The problem for me was that my stream was not set to the origin before download.
Response.Clear();
Response.ContentType = "Application/msword";
Response.AddHeader("Content-Disposition", "attachment; filename=myfile.docx");
//ADDED THIS LINE
myMemoryStream.Seek(0,SeekOrigin.Begin);
myMemoryStream.WriteTo(Response.OutputStream);
Response.Flush();
Response.Close();