需求描述：在PDF檔案上的某個指定位置顯示指定內容的二維碼，其中，位置為PDF的四個角（LU , RU, RD, LD) ；座標為相對這四個角的座標，即LU(50,50)指二維碼的左上角相對PDF左上角的偏移(50,50); RD(50,50)即二維碼的右下角相對PDF檔案右下角的偏移(50,50)，以此類推。包括二維碼內容都由使用者在頁面上配置。在用itextsharp顯示二維碼的時候，需要的是二維碼左下角相對PDF左下角的偏移，所以需要寫一個座標變換把使用者的配置資訊統一轉換到itextsharp顯示需要的座標系下。
定義LD為基準座標系，可用 (1001)$\left(\begin{array}{cc}1& 0\\ 0& 1\end{array}\right)$表示，則RD為(100−1)$\left(\begin{array}{cc}1& 0\\ 0& -1\end{array}\right)$，RU為

(−100−1)$\left(\begin{array}{cc}-1& 0\\ 0& -1\end{array}\right)$，LU為(−1001)$\left(\begin{array}{cc}-1& 0\\ 0& 1\end{array}\right)$，設給出的座標是LU(pos(x,y))，則有 pos’ = pos * matrix[‘LU’]，然後加上LD座標系下的PDF尺寸就是LD下的座標啦，還要變換成二維碼的左下角座標，再加上二維碼的尺寸搞定。程式碼裡寫了好多註釋就怕自己搞暈了，哈哈！（順便感謝何燕傑這篇部落格裡的c#矩陣運算類庫）
唯一的缺陷就是會生成一個新的PDF檔案，暫時不影響專案就沒多在這裡浪費時間了，如果有人知道怎麼可以不需要頻繁的delete、rename或moveto操作下不生成新的PDF檔案而完成任務請告知，謝了先！主要還是對itextsharp不熟吧。
========================我是找不到物件就自己new的分割線======================

    public class PdfQRHelper
    {
        #region 座標系變換
        public enum BasePointName { LU = 0, RU, RD, LD };
        private static readonly Dictionary<BasePointName, Matrix> CoordinateBasis = new Dictionary<BasePointName, Matrix>()
        {
            {BasePointName.LD, new double
 
[,] { {1, 0}, {0, 1} } },
            {BasePointName.RD, new double[,] { {-1, 0},{0, 1} } },
            {BasePointName.LU, new double[,] { {1, 0}, {0, -1} } },
            {BasePointName.RU, new double[,] { {-1, 0}, {0, -1} } }
        };

        //變換到目標座標系下後的數值修正係數矩陣，與BasePointName順序一一對應
        private static readonly Matrix SizeFixMatrix = new double[,] 
        {
            {0, 1},
            {1, 1},
            {1, 0},
            {0, 0}
        };

        /// <summary>
        /// 座標變換，將原正交基定義下的座標變換到[[1,0],[0,1]]定義的座標下
        /// </summary>
        /// <param name="srcBasisName">原始正交基</param>
        /// <param name="srcPosition">原始正交基下座標</param>
        /// <param name="destBasisSize">目標座標系的[w,h], 行向量</param>
        /// <returns>返回[[1,0],[0,1]]正交基下的座標</returns>
        public static Vector CoordinateConvert(BasePointName srcBasisName, Vector srcPosition, Vector destBasisSize)
        {
            Matrix sizeFixMat = Dot(destBasisSize, SizeFixMatrix);
            Matrix srcPosMat = Matrix.Create(1, srcPosition.Count, srcPosition.Elements);            
            Vector resVec = new Vector((srcPosMat * CoordinateBasis[srcBasisName]).GetRow(0).ToArray(), VectorType.Row);
            Vector fixVec = new Vector(sizeFixMat.GetRow((int)srcBasisName).ToArray(), VectorType.Row);
            return resVec + fixVec;

        }

        /// <summary>
        /// 向量和矩陣做點乘
        /// </summary>
        /// <param name="vec">運算的向量，行向量</param>
        /// <param name="matrix">運算的矩陣</param>
        /// <returns>返回點乘後的矩陣</returns>
        private static Matrix Dot(Vector vec, Matrix matrix)
        {
            Matrix mat = new Matrix(matrix);
            if(vec.Count != matrix.ColumnCount)
            {
                throw new Exception("the colum must be equal!");                
            }

            for(int i = 0; i < matrix.RowCount; i++)
            {
                for (int j = 0; j<matrix.ColumnCount;j++)
                {
                    mat[i, j] = vec[j] * matrix[i, j];
                }
            }

            return mat;
        }
        #endregion

        #region 生成二維碼
        /// <summary>
        /// 將相對四個角的座標統一換算成二維碼右下角的座標，座標系轉換前呼叫
        /// </summary>
        /// <param name="basePoint">基座標系</param>
        /// <param name="relativePos">相對四個角的座標</param>
        /// <param name="qrSize">二維碼大小</param>
        /// <returns>返回二維碼右下角座標</returns>
        private static double[] GetQRanderPos(PdfQRHelper.BasePointName basePoint, double[] relativePos, double[] qrSize)
        {
            double qrW = qrSize[0];
            double qrH = qrSize[1];
            double[] absPos = relativePos;
            switch (basePoint)
            {
                case PdfQRHelper.BasePointName.LU:
                    absPos[1] += qrSize[1];
                    break;
                case PdfQRHelper.BasePointName.RU:
                    absPos[0] += qrSize[0];
                    absPos[1] += qrSize[1];
                    break;
                case PdfQRHelper.BasePointName.RD:
                    absPos[0] += qrSize[0];
                    break;
            }
            return absPos;
        }

        /// <summary>
        /// 生成二維碼
        /// </summary>
        /// <param name="content">需要生成二維碼的內容</param>
        /// <param name="size">二維碼圖片長寬大小</param>
        /// <returns></returns>
        public static Bitmap CreateQRbmp(string content, int size)
        {
            try
            {
                var options = new QrCodeEncodingOptions
                {
                    DisableECI = true,
                    CharacterSet = "UTF-8",
                    Width = size,
                    Height = size,
                    Margin = 0,
                    ErrorCorrection = ZXing.QrCode.Internal.ErrorCorrectionLevel.H
                };
                var writer = new BarcodeWriter();
                writer.Format = BarcodeFormat.QR_CODE;
                writer.Options = options;
                var bmp = writer.Write(content);
                return bmp;
            }
            catch (Exception ex)
            {
                return null;
            }
        }

        private static double[] GetQRPosition(PdfQRHelper.BasePointName basePoint, double[] srcPos,float[] pdfSize)
        {            
            float height = pdfSize[1];
            float width = pdfSize[0]; 
            Vector srcSize = new Vector(new double[] { width, height }, VectorType.Row);
            Vector srcPosition = new Vector(srcPos, VectorType.Row);
            Vector pos = CoordinateConvert(basePoint, srcPosition, srcSize);            
            return pos.Elements;
        }

        /// <summary>
        /// 在原PDF檔案上的指定位置新增指定內容的二維碼
        /// </summary>
        /// <param name="srcPdfPath">原PDF檔案</param>
        /// <param name="qrContent">二維碼內容</param>
        /// <param name="qrSize">二維碼大小，單位：同PDF尺寸單位</param>
        /// <param name="qrRelPos">二維碼相對四個角的位置，double[]型別</param>
        /// <param name="basePointName">PDF四個角列舉值，預設左下</param>
        /// <returns>在同一目錄下返回帶二維碼的新PDF檔案，檔名：原始檔名_qr.pdf,失敗返回null</returns>
        public static string GenerateQRInPdf(string srcPdfPath,string qrContent, int qrSize,double[] qrRelPos,PdfQRHelper.BasePointName basePointName = BasePointName.LD)
        {
            string tempPdf = string.Format("{0}_qr.pdf", System.IO.Path.GetFileNameWithoutExtension(srcPdfPath));
            PdfReader pdfReader = null;
            PdfStamper stamper = null;
            try
            {
                pdfReader = new PdfReader(srcPdfPath);
                iTextSharp.text.Rectangle pageSize = pdfReader.GetPageSize(1);                
                stamper = new PdfStamper(pdfReader,new System.IO.FileStream(tempPdf, System.IO.FileMode.OpenOrCreate));
                PdfContentByte canvas = stamper.GetOverContent(1);
                string url = qrContent;                
                var qrImg = CreateQRbmp(url, qrSize);
                iTextSharp.text.Image image = iTextSharp.text.Image.GetInstance(qrImg, System.Drawing.Imaging.ImageFormat.Bmp);
                double[] absPos = GetQRanderPos(basePointName, qrRelPos, new double[] { qrSize, qrSize });
                double[] desPos = GetQRPosition(basePointName, absPos, new float[] { pageSize.Width, pageSize.Height });
                image.SetAbsolutePosition((float)desPos[0], (float)desPos[1]); // 新增二維碼圖片位置
                canvas.AddImage(image);                
            }
            catch (Exception ex)
            {
                return null;
            }
            finally
            {
                if (stamper != null)
                {
                    stamper.Close();
                    stamper = null;
                }
                if (pdfReader != null)
                {
                    pdfReader.Close();
                    pdfReader = null;
                }                              
            }
            return tempPdf;
        }        
        #endregion
    }