空白PDF生成时，WriteCompatiblePdf方法调用ITextSharp

你好，我使用下面的代码。当我上传一些包含一些内容的PDF文件时，上传后，上传的PDF文件上没有内容。上传的PDf为空白。我正在使用下面的ItextSharp方法将原始PDF版本更改为定义版本。

private int WriteCompatiblePdf(string fileName, FileUpload filePath)
        {
            string sNewPdf = HttpContext.Current.Server.MapPath(ConfigurationManager.AppSettings["InterfaxPath"]) + fileName;
            iTextSharp.text.pdf.PdfReader reader = new iTextSharp.text.pdf.PdfReader(filePath.FileBytes);
            // we retrieve the total number of pages
            int n = reader.NumberOfPages;
            // step 1: creation of a document-object
            iTextSharp.text.Document document = new iTextSharp.text.Document(reader.GetPageSizeWithRotation(1));
            // step 2: we create a writer that listens to the document
            iTextSharp.text.pdf.PdfWriter writer = iTextSharp.text.pdf.PdfWriter.GetInstance(document, new FileStream(sNewPdf, FileMode.Create, FileAccess.ReadWrite));
            //write pdf that pdfsharp can understand
            writer.SetPdfVersion(iTextSharp.text.pdf.PdfWriter.PDF_VERSION_1_4);
            // step 3: we open the document
            document.Open();
            iTextSharp.text.pdf.PdfContentByte cb = writer.DirectContent;
            iTextSharp.text.pdf.PdfImportedPage page;
            int rotation;
            int i = 0;
            while (i < n)
            {
                i++;
                document.SetPageSize(reader.GetPageSizeWithRotation(i));
                document.NewPage();
                page = writer.GetImportedPage(reader, i);
                rotation = reader.GetPageRotation(i);                    
                cb.AddTemplate(page, 1f, 0, 0, 1f, 0, 0);                    
            }
            // step 5: we close the document
            document.Close();
            return n;
        }

建议吗?