import com.spire.pdf.FileFormat;
import com.spire.pdf.PdfDocument;
import com.spire.pdf.PdfPageBase;
import com.spire.pdf.graphics.*;

import java.awt.*;
import java.awt.geom.Dimension2D;
import java.awt.geom.Point2D;
import java.awt.geom.Rectangle2D;
import java.io.ByteArrayInputStream;


public class PdfWatermarkDemo {

    public void addWatermark(String pdfFile, String watermarkType, String text, byte[] imageData){
        String outputFile = "output.pdf";

        PdfDocument pdf=new PdfDocument();
        pdf.loadFromFile(pdfFile);
        switch (watermarkType) {
            case "Text":
                addTextWatermark(pdf, text);
                break;
            case "Image":
                addImageWatermark(pdf, imageData);
                break;
        }
        pdf.saveToFile(outputFile, FileFormat.PDF);
        pdf.close();
    }
    public void addImageWatermark(PdfDocument pdf, byte[] imgData) {
        PdfImage image = PdfImage.fromStream(new ByteArrayInputStream(imgData));
        for (int i = 0; i < pdf.getPages().getCount(); i++) {
            PdfPageBase page = pdf.getPages().get(i);
            page.getCanvas().save();
            page.getCanvas().setTransparency(0.5f, 0.5f, PdfBlendMode.Multiply);
            page.getCanvas().drawImage(image, new Point2D.Float(160, 260));
            page.getCanvas().restore();
        }
    }
    public void addTextWatermark(PdfDocument pdf, String text) {
        for (int i = 0; i < pdf.getPages().getCount(); i++) {
            PdfPageBase page = pdf.getPages().get(i);
            Dimension2D dimension2D = new Dimension();
            dimension2D.setSize(page.getCanvas().getClientSize().getWidth() / 2, page.getCanvas().getClientSize().getHeight() / 3);
            PdfTilingBrush brush = new PdfTilingBrush(dimension2D);
            brush.getGraphics().setTransparency(0.3F);
            brush.getGraphics().save();
            brush.getGraphics().translateTransform((float) brush.getSize().getWidth() / 2, (float) brush.getSize().getHeight() / 2);
            brush.getGraphics().rotateTransform(-45);
            brush.getGraphics().drawString(text, new PdfTrueTypeFont(new Font("Arial", Font.BOLD, 80), true), PdfBrushes.getViolet(), 0, 0, new PdfStringFormat(PdfTextAlignment.Center));
            brush.getGraphics().restore();
            brush.getGraphics().setTransparency(0.5);
            Rectangle2D loRect = new Rectangle2D.Float();
            loRect.setFrame(new Point2D.Float(0, 0), page.getCanvas().getClientSize());
            page.getCanvas().drawRectangle(brush, loRect);
        }
    }
}

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

namespace DemoOnlineCode
{
    class WatermarkDemo
    {
        public void addWatermark(string pdfFile, string watermarkType, string text, byte[] imageData)
        {
            string outputFile = "output.pdf";

            PdfDocument pdf = new PdfDocument();
            pdf.LoadFromFile(pdfFile);
            switch (watermarkType)
            {
                case "Text":
                    addTextWatermark(pdf, text);
                    break;
                case "Image":
                    addImageWatermark(pdf, imageData);
                    break;
            }
            pdf.SaveToFile(outputFile, FileFormat.PDF);
            pdf.Close();
        }
        public void addImageWatermark(PdfDocument pdf, byte[] imgData)
        {
            PdfImage image = PdfImage.FromStream(new MemoryStream(imgData));
            for (int i = 0; i < pdf.Pages.Count; i++)
            {
                PdfPageBase page = pdf.Pages[i];
                page.Canvas.Save();
                page.Canvas.SetTransparency(0.5f, 0.5f, PdfBlendMode.Multiply);
                page.Canvas.DrawImage(image, new System.Drawing.PointF(160,260));
                page.Canvas.Restore();
            }
        }
        public void addTextWatermark(PdfDocument pdf, string text)
        {
            for (int i = 0; i < pdf.Pages.Count; i++)
            {
                PdfPageBase page = pdf.Pages[i];

                PdfTilingBrush brush = new PdfTilingBrush(new SizeF(page.Canvas.ClientSize.Width / 2, page.Canvas.ClientSize.Height / 3));
                brush.Graphics.SetTransparency(0.3F);
                brush.Graphics.Save();
                brush.Graphics.TranslateTransform(brush.Size.Width / 2, brush.Size.Height / 2);
                brush.Graphics.RotateTransform(-45);
                brush.Graphics.DrawString(text, new PdfTrueTypeFont(new Font("Arial",80,FontStyle.Bold), true), PdfBrushes.Violet, 0, 0, new PdfStringFormat(PdfTextAlignment.Center));
                brush.Graphics.Restore();
                brush.Graphics.SetTransparency(0.5f);
                page.Canvas.DrawRectangle(brush, new RectangleF(new PointF(0, 0), page.Canvas.ClientSize));
            }
        }
    }
}