C#使用iTextSharp压缩PDF图片

实现思路：

1、使用NuGet获取iTextShart引用

2、使用iTextSharp读取PDF文档，遍历每一页中的图片,循环每一个图片;

3、使用图片压缩算法，根据自己需要，压缩满足条件的图片(例子中是限度是宽和高中的最小值大于1080的才压缩);

4、将压缩后的图片写回到PDF中

具体代码如下：

    /// <summary>
    /// PDF文件压缩处理,只针对PDF文件里的图片
    /// </summary>
    public class PDFCompression
    {
        /// <summary>
        /// 压缩PDF文件里的图片
        /// </summary>
        /// <param name="sourceFile"></param>
        /// <param name="TargetFile"></param>
        public static void CompressPDFFile(string sourceFile, string TargetFile)
        {
            if (new FileInfo(sourceFile).Length <= 5 * 1024 * 1024)
            {
                File.Copy(sourceFile, TargetFile);
            }
            else
            {
                try
                {
                    PdfReader reader2 = new PdfReader(sourceFile);
                    //输出文件流.
                    using (FileStream fs = new FileStream(TargetFile, FileMode.Create, FileAccess.Write, FileShare.None))
                    {
                        using (PdfStamper stamper = new PdfStamper(reader2, fs))
                        {
                            PdfDictionary page;

                            //Get the page count
                            int pageCount2 = reader2.NumberOfPages;
                            //Loop through each page
                            for (int i = 1; i <= pageCount2; i++)
                            {
                                //Get the page
                                page = reader2.GetPageN(i);
                                var pdfObjects = FindImageInPDFDictionarys(page, DistinguishImageIsNeedCompress);
                                foreach (var obj in pdfObjects)
                                {
                                    if (obj != null)
                                    {
                                        iTextSharp.text.Image img = ResizedImage(obj, reader2);
                                        iTextSharp.text.Image maskImage = img.ImageMask;
                                        PdfReader.KillIndirect(obj);//移除老图片,只是移除了关联. 
                                        if (maskImage != null)
                                            stamper.Writer.AddDirectImageSimple(maskImage); //把新图片写进去
                                        if (img != null)
                                            stamper.Writer.AddDirectImageSimple(img, (PRIndirectReference)obj); //把新图片写进去.
                                    }
                                }
                            }

                        }
                    }
                }
                catch (Exception ex)
                {
                    File.Copy(sourceFile, TargetFile, true);
                }
            }
        }

        //在pdf页面中 找到图片
        private static List<PdfObject> FindImageInPDFDictionarys(PdfDictionary pg, DistinguishImage distinguishMethod)
        {
            List<PdfObject> pdfObjects = new List<PdfObject>();
            try
            {
                PdfDictionary res = (PdfDictionary)PdfReader.GetPdfObject(pg.Get(PdfName.RESOURCES));
                PdfDictionary xobj = (PdfDictionary)PdfReader.GetPdfObject(res.Get(PdfName.XOBJECT));
                if (xobj != null)
                {
                    foreach (PdfName name in xobj.Keys)
                    {
                        Console.WriteLine(name.ToString());
                        PdfObject obj = xobj.Get(name);
                        if (obj.IsIndirect())
                        {
                            PdfDictionary tg = (PdfDictionary)PdfReader.GetPdfObject(obj);
                            PdfName type = (PdfName)PdfReader.GetPdfObject(tg.Get(PdfName.SUBTYPE));
                            //image at the root of the pdf
                            if (PdfName.IMAGE.Equals(type))
                            {
                                if (distinguishMethod(tg) == true)
                                {
                                    pdfObjects.Add(obj);
                                }
                                else
                                {
                                    continue;//继续找
                                }
                            }
                            //// image inside a form
                            //else if (PdfName.FORM.Equals(type))
                            //{
                            //    return FindImageInPDFDictionarys(tg, distinguishMethod);
                            //}
                            ////image inside a group
                            //else if (PdfName.GROUP.Equals(type))
                            //{
                            //    return FindImageInPDFDictionarys(tg, distinguishMethod);
                            //}
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                pdfObjects = new List<PdfObject>();
            }
            return pdfObjects;
        }

        /// <summary>
        /// 辨别图片的委托
        /// </summary>
        /// <param name="imgObject"></param>
        /// <returns></returns>
        delegate bool DistinguishImage(PdfDictionary imgObject);

        /// <summary>
        /// 辨别图片是不是需要压缩
        /// </summary>
        /// <param name="imgObject"></param>
        /// <returns></returns>
        private static bool DistinguishImageIsNeedCompress(PdfDictionary imgObject)
        {
            //int width, height, length;
            //int.TryParse(imgObject.Get(PdfName.WIDTH).ToString(), out width);
            //int.TryParse(imgObject.Get(PdfName.HEIGHT).ToString(), out height);
            //int.TryParse(imgObject.Get(PdfName.LENGTH).ToString(), out length);

            ////从这3个参数就可以判断是不是需要压缩.
            //if (width == 270 && height == 111 && length == 11878)
            //{
            //    return true;
            //}
            //else
            //{
            //    return false;
            //}
            return true;
        }

        private static iTextSharp.text.Image ResizedImage(PdfObject obj, PdfReader reader)
        {
            if (obj != null)
            {
                int XrefIndex =
                    Convert.ToInt32(((PRIndirectReference)obj).Number.ToString(System.Globalization.CultureInfo
                        .InvariantCulture));
                PdfObject pdfObj = reader.GetPdfObject(XrefIndex);
                PdfStream pdfStrem = (PdfStream)pdfObj;
                byte[] bytes = PdfReader.GetStreamBytesRaw((PRStream)pdfStrem);
                if ((bytes != null))
                {
                    using (System.IO.MemoryStream memStream = new System.IO.MemoryStream(bytes))
                    {
                        memStream.Position = 0;
                        System.Drawing.Image img = ImageHelper.ResizeImage(System.Drawing.Image.FromStream(memStream), .4M);
                        ImageFormat format = img.PixelFormat == PixelFormat.Format1bppIndexed
                                             || img.PixelFormat == PixelFormat.Format4bppIndexed
                                             || img.PixelFormat == PixelFormat.Format8bppIndexed
                            ? ImageFormat.Tiff
                            : ImageFormat.Jpeg;

                        var pdfImage = iTextSharp.text.Image.GetInstance(img, format);
                        return pdfImage;
                    }
                }
                else
                {
                    return null;
                }
            }
            else
            {
                return null;
            }
        }

        public static Bitmap BitmapToGrayscale(Bitmap source)
        {
            // Create target image.
            int width = source.Width;
            int height = source.Height;
            Bitmap target = new Bitmap(width, height, PixelFormat.Format8bppIndexed);
            // Set the palette to discrete shades of gray
            ColorPalette palette = target.Palette;
            for (int i = 0; i < palette.Entries.Length; i++)
            {
                palette.Entries[i] = Color.FromArgb(0, i, i, i);
            }
            target.Palette = palette;

            // Lock bits so we have direct access to bitmap data
            BitmapData targetData = target.LockBits(new System.Drawing.Rectangle(0, 0, width, height),
                ImageLockMode.ReadWrite, PixelFormat.Format8bppIndexed);
            BitmapData sourceData = source.LockBits(new System.Drawing.Rectangle(0, 0, width, height),
                ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb);

            unsafe
            {
                for (int r = 0; r < height; r++)
                {
                    byte* pTarget = (byte*)(targetData.Scan0 + r * targetData.Stride);
                    byte* pSource = (byte*)(sourceData.Scan0 + r * sourceData.Stride);
                    for (int c = 0; c < width; c++)
                    {
                        byte colorIndex = (byte)(((*pSource) * 0.3 + *(pSource + 1) * 0.59 + *(pSource + 2) * 0.11));
                        *pTarget = colorIndex;
                        pTarget++;
                        pSource += 3;
                    }
                }
            }

            target.UnlockBits(targetData);
            source.UnlockBits(sourceData);
            return target;
        }
        /// <summary>
        /// 判断是否PDF文件
        /// </summary>
        /// <param name="filePath"></param>
        /// <returns></returns>
        public static bool IsPDF(string filePath)
        {
            try
            {
                return Path.GetExtension(filePath).ToLower().Contains("pdf");
            }
            catch(Exception ex)
            {
                return false;
            }
        }
    }

    public static class ImageHelper
    {
        /// <summary>
        /// Resize the image to the specified width and height.
        /// </summary>
        /// <param name="image">The image to resize.</param>
        /// <param name="width">The width to resize to.</param>
        /// <param name="height">The height to resize to.</param>
        /// <returns>The resized image.</returns>
        public static Bitmap ResizeImage(System.Drawing.Image image, int width, int height)
        {
            var destRect = new System.Drawing.Rectangle(0, 0, width, height);
            var destImage = new System.Drawing.Bitmap(width, height);

            destImage.SetResolution(image.HorizontalResolution, image.VerticalResolution);

            using (var graphics = Graphics.FromImage(destImage))
            {
                graphics.CompositingMode = CompositingMode.SourceCopy;
                graphics.CompositingQuality = CompositingQuality.HighQuality;
                graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
                graphics.SmoothingMode = SmoothingMode.HighQuality;
                graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;

                using (var wrapMode = new ImageAttributes())
                {
                    wrapMode.SetWrapMode(WrapMode.TileFlipXY);
                    graphics.DrawImage(image, destRect, 0, 0, image.Width, image.Height, GraphicsUnit.Pixel, wrapMode);
                }
            }

            return destImage;
        }

        public static Bitmap ResizeImage(System.Drawing.Image image, decimal percentage)
        {
            int baseSize = image.Height;
            if (image.Width < image.Height)
            {
                baseSize = image.Width;
            }

            int width = image.Width;
            int height = image.Height;

            if (baseSize > 1080)
            {
                width = (int)Math.Round(image.Width * percentage, MidpointRounding.AwayFromZero);
                height = (int)Math.Round(image.Height * percentage, MidpointRounding.AwayFromZero);
            }

            return ResizeImage(image, width, height);
        }
    }

　　

参考链接：

https://github.com/jpeckham/CompressPDF

https://blog.csdn.net/phker/article/details/92806191

https://kb.itextpdf.com/home/it7kb/ebooks/itext-7-jump-start-tutorial-for-net