Install-Package Spire.PDF

using Spire.Pdf;
using Spire.Pdf.Graphics;
using System.Drawing;

namespace AddTextWatermark
{
    class Program
    {
        static void Main(string[] args)
        {
            // 加載文檔
            PdfDocument doc = new PdfDocument();
            doc.LoadFromFile("C:\\Users\\Administrator\\Desktop\\AI.pdf");

            // 創(chuàng)建字體和畫刷
            PdfTrueTypeFont font = new PdfTrueTypeFont(new Font("Arial Black", 50f), true);
            PdfBrush brush = PdfBrushes.Blue;

            // 指定水印文字
            string watermarkText = "DO NOT COPY";

            // 設置透明度
            float opacify = 0.6f;

            // 遍歷頁
            foreach (PdfPageBase page in doc.Pages)
            {
                // 繪制水印文字
                AddWatermark(page, watermarkText, font, brush, opacify);
            }

            // 保存文檔
            doc.SaveToFile("Watermark.pdf");
            doc.Dispose();
        }

        private static void AddWatermark(PdfPageBase page, string watermarkText, PdfTrueTypeFont font, PdfBrush brush, float opacity)
        {
            page.Canvas.SetTransparency(opacity);
            SizeF textSize = font.MeasureString(watermarkText);
            float pageWidth = page.ActualSize.Width;
            float pageHeight = page.ActualSize.Height;
            float x = (pageWidth - textSize.Width) / 2;
            float y = (pageHeight - textSize.Height) / 2;
            
            //page.Canvas.RotateTransform(-45);
            page.Canvas.DrawString(watermarkText, font, brush, x, y);
        }
    }
}

using Spire.Pdf;
using System.Drawing;

namespace AddImageWatermark
{
    class Program
    {
        static void Main(string[] args)
        {
            //創(chuàng)建一個PdfDocument對象
            PdfDocument document = new PdfDocument();

            //加載示例PDF文檔
            document.LoadFromFile(@"C:\Users\Administrator\Desktop\sample.pdf");

            //加載圖片
            Image image = Image.FromFile(@"C:\Users\Administrator\Desktop\logo.png");

            //獲取圖片寬度和高度
            int imgWidth = image.Width;
            int imgHeight = image.Height;

            //遍歷頁面
            for (int i = 0; i < document.Pages.Count; i++)
            {
                //獲取頁面寬度和高度
                float pageWidth = document.Pages[i].ActualSize.Width;
                float pageHeight = document.Pages[i].ActualSize.Height;

                //設置背景不透明度
                document.Pages[i].BackgroudOpacity = 0.3f;

                //設置當前頁面的背景圖片
                document.Pages[i].BackgroundImage = image;

                //將背景圖片置于頁面中央
                Rectangle rect = new Rectangle((int)(pageWidth - imgWidth) / 2, (int)(pageHeight - imgHeight) / 2, imgWidth, imgHeight);
                document.Pages[i].BackgroundRegion = rect;
            }
            //保存文檔
            document.SaveToFile("AddImageWatermark.pdf");
            document.Close();
        }
    }
}