iTextSharp文件操作和显示到FileStreamResults

我正在尝试使用iTextSharp打开PDF文件，向文件添加数据集以预填充数据，然后将其保存到流中，以便我可以将其显示给用户。我不想把它保存到本地文件中。我一直得到错误"无法访问已关闭的流。"我不知道哪个流是错的。

下面是我的代码:

public FileStreamResult PushDataIntoPDFStream(string filename)
{     
     var reader = new PdfReader(Path.Combine(Server.MapPath(path), filename));
     var xml = @"<?xml version=""1.0"" encoding=""UTF-8""?>
          <form1>  
             <firstName>test</firstName>
             <lastName>user</lastName>
             <driveCar>0</driveCar>
             <gender>1</gender>
             <birthdate>2011-08-12</birthdate>
             <numPets>4</numPets>
          </form1>";
     using (var outstream = new MemoryStream())
     { 
          using (var stamper = new PdfStamper(reader, outstream))
          {
               var bytes = System.Text.Encoding.UTF8.GetBytes(xml);
               using (var ms = new MemoryStream(bytes))
               {
                    stamper.AcroFields.Xfa.FillXfaForm(ms);
               }
          }
          return new FileStreamResult(outstream, "application/pdf")
          {
                FileDownloadName = "file.pdf";
          };
     }      
  }

using (var stamper = new PdfStamper(reader, outstream))
{
    stamper.Writer.CloseStream = false;