public void SetPdfBackground(string pdfFilePath) 
{
 //重新生成的 PDF 的路徑
 string destFile = HttpContext.Current.Server.MapPath("sample.pdf");
 //create new pdf document
 FileStream stream = new FileStream(destFile, FileMode.Create, FileAccess.ReadWrite);
 PdfReader reader = new PdfReader(pdfFilePath);
 //read pdf stream 
 PdfStamper stamper = new PdfStamper(reader, stream);
 string imagePage = HttpContext.Current.Server.MapPath("../images/2012/bg2.png");
 System.Drawing.Image image = System.Drawing.Image.FromFile(imagePage);
 var img = Image.GetInstance(image, System.Drawing.Imaging.ImageFormat.Png);
 img.SetAbsolutePosition(0, 0);
 int totalPage = reader.NumberOfPages;
 for (int current = 1; current <= totalPage; current++)
 {
  var canvas = stamper.GetUnderContent(current);
  var page = stamper.GetImportedPage(reader, current);
  canvas.AddImage(img);
 }
 stamper.Close();
 reader.Close();
}
 

using System.Drawing;
using Spire.Pdf; 

PdfDocument doc = new PdfDocument();
doc.LoadFromFile("sample.pdf");

PdfPageBase page = doc.Pages[0];

Image backgroundImage = Image.FromFile("background.jpg");
page.BackgroundImage = backgroundImage;

doc.SaveToFile("result.pdf");
System.Diagnostics.Process.Start("result.pdf");

using System.Drawing;
using Spire.Pdf;
 
namespace Add_image_background_to_PDF
{
  class Program
  {
    static void Main(string[] args)
    {
      PdfDocument doc = new PdfDocument();
      doc.LoadFromFile("sample.pdf");
      PdfPageBase page = doc.Pages[0];
      Image backgroundImage = Image.FromFile("background.jpg");
      page.BackgroundImage = backgroundImage;
 
      doc.SaveToFile("result.pdf");
      System.Diagnostics.Process.Start("result.pdf");
    }
  }
}