使用PdfSharpCore请注意使用XGraphics基类，与System.Drawing 的Graphics类似，XGraphics 提供XColor（颜色）、XPen（画笔）、XBrush（画刷）、XFont(字体)、XPoint(位置)等对象。提供很多画线，矩形，圆，扇形，多边形，图，文本等方法。

源码请查看:https://github.com/ststeiger/PdfSharpCore/blob/master/PdfSharpCore/Drawing/XGraphics.cs

1.设置PDF拥有者的密码，让PDF防篡改
代码很简单设置PdfDocument.SecuritySettings.OwnerPassword 

PdfDocument doc = PdfReader.Open(@"a.pdf", PdfDocumentOpenMode.Modify);
doc.SecuritySettings.OwnerPassword = "123";
var filePath = $"b.pdf";
doc.Save(filePath);

2.PDF添加页眉和页脚
（1）添加页码显示
 XStringFormats 指定文本的位置：详请查看https://github.com/ststeiger/PdfSharpCore/blob/master/PdfSharpCore/Drawing/XStringFormats.cs

         XFont font = new XFont("SimHei", 8);
            XBrush brush = XBrushes.Black;
            PdfDocument doc = PdfReader.Open(@"a.pdf", PdfDocumentOpenMode.Modify);
            for (int i = 0; i < doc.Pages.Count; i++)
            {
                PdfPage page = doc.Pages[i];
                XRect layoutRectangle = new XRect(0, page.Height - font.Height, page.Width, font.Height);
                using (XGraphics gfx = XGraphics.FromPdfPage(page))
                {
                    gfx.DrawString(
                        $"第{(i + 1).ToString()}页/共{doc.Pages.Count}页",
                        font,
                        brush,
                        layoutRectangle,
                        XStringFormats.BottomLeft);
                }
            }

（2）添加页眉

     XFont font = new XFont("SimHei", 8);
            XBrush brush = new XSolidBrush(XColor.FromArgb(128, 255, 0, 0));
            XPoint point = new XPoint(90, 20);
            PdfDocument doc = PdfReader.Open(@"a.pdf", PdfDocumentOpenMode.Modify);
            for (int i = 0; i < doc.Pages.Count; i++)
            {
                var renderer = XGraphics.FromPdfPage(doc.Pages[i]);
                XSize pageSize = renderer.PageSize;
                renderer.DrawString("xxx有限公司", font, brush, point);
                XPen pen = new XPen(XBrushes.Gray, 0.5f);
                renderer.DrawLine(pen, point.X, point.Y, pageSize.Width - point.X, point.Y);
            }
            doc.Save("b.pdf");

（3）添加页脚

            XFont font = new XFont("SimHei", 8);
            XBrush brush = new XSolidBrush(XColor.FromArgb(128, 255, 0, 0));
            PdfDocument doc = PdfReader.Open(@"a.pdf", PdfDocumentOpenMode.Modify);
            for (int i = 0; i < doc.Pages.Count; i++)
            {
                var renderer = XGraphics.FromPdfPage(doc.Pages[i]);
                XSize pageSize = renderer.PageSize;
                XPoint point = new XPoint(90, pageSize.Height-20);
                renderer.DrawString("xxx有限公司", font, brush, point);
                XPen pen = new XPen(XBrushes.Gray, 0.5f);
                renderer.DrawLine(pen, point.X, point.Y-10, pageSize.Width - point.X, point.Y-10);
            }
            doc.Save("b.pdf");

3.PDF添加水印文字

   XFont font = new XFont("SimHei", 8);
            XBrush brush =new XSolidBrush(XColor.FromArgb(128, 255, 0, 0));
            PdfDocument doc = PdfReader.Open(@"a.pdf", PdfDocumentOpenMode.Modify);
            for (int i = 0; i < doc.Pages.Count; i++)
            {
                XStringFormat stringFormat = new XStringFormat();
                stringFormat.Alignment = XStringAlignment.Center;
                stringFormat.LineAlignment = XLineAlignment.Center;
                PdfPage page = doc.Pages[i];
                var gfx = XGraphics.FromPdfPage(page, XPageDirection.Downwards);
                gfx.DrawString(
                       $"xxx公司版权所有",
                       font,
                       brush,
                       new XPoint(500, 500),
                        stringFormat);
            }
            doc.Save("b.pdf");

 4.PDF 添加图片

    //第一步先加载PDF文件
            PdfDocument doc = PdfReader.Open(@"a.pdf", PdfDocumentOpenMode.Modify);
            //导入图片（地址，文件流）
            var background = XImage.FromFile(@"QRCode.png");
            // var background = XImage.FromStream(()=> stream);
            //指定PDF 的页
            PdfPage page = doc.Pages[0];
            var gfx = XGraphics.FromPdfPage(page, XPageDirection.Downwards);
            //写入指定位置
            gfx.DrawImage(background, 20, 20, 250, 140);
            doc.Save("b.pdf");