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