只是一个狂野而疯狂的猜测，但我想你之所以如此创建新文件就是这一行。

Just a wild and crazy guess, but I think the reason why you are always creating new files is this line.

PdfWriter.GetInstance(doc, new FileStream(pdfpath, FileMode.Create));

您正在使用FileMode.Create ...您应该将其更改为FileMode.OpenOrCreate或FileMode.Open。​​

You are using FileMode.Create...you should probably change that to FileMode.OpenOrCreate or FileMode.Open.

请试试这个

Please try this

using (Stream inputPdfStream = new FileStream(Server.MapPath("~") + "/pdf/test.pdf", FileMode.Open, FileAccess.Read, FileShare.Read))
           using (Stream inputImageStream = new FileStream(Server.MapPath("~") + "/logo/test.jpg", FileMode.Open, FileAccess.Read, FileShare.Read))
           using (Stream outputPdfStream = new FileStream(Server.MapPath("~") + "/pdf/Result.pdf", FileMode.Create, FileAccess.Write, FileShare.None))
           {
               var reader = new PdfReader(inputPdfStream);
               var stamper = new PdfStamper(reader, outputPdfStream);
               var pdfContentByte = stamper.GetOverContent(1);

               iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance(inputImageStream);
               image.SetAbsolutePosition(100, 100);
               pdfContentByte.AddImage(image);
               stamper.Close();
           }