Hey Vinoth548,

Please refer below sample for adding watermark.

For adding footer refer below article.

iTextSharp: Add Page numbers to existing PDF using C# and VB.Net

Namespaces

C#

using iTextSharp.text.pdf;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;

VB.Net

Imports iTextSharp.text.pdf
Imports System.IO
Imports System.Drawing.Imaging
Imports System.Drawing

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    PdfStampInExistingFile("@aspsnippets.com");
}

private void PdfStampInExistingFile(string text)
{
    string sourceFilePath = @"C:\Users\anand\Desktop\Test.pdf";
    byte[] bytes = File.ReadAllBytes(sourceFilePath);
    Bitmap bitmap = new Bitmap(200, 30, System.Drawing.Imaging.PixelFormat.Format64bppArgb);
    Graphics graphics = Graphics.FromImage(bitmap);
    graphics.Clear(Color.White);
    graphics.DrawString(text, new System.Drawing.Font("Arial", 12, FontStyle.Bold), new SolidBrush(Color.Red), new PointF(0.4F, 2.4F));
    bitmap.Save(Server.MapPath("~/Image.jpg"), ImageFormat.Jpeg);
    bitmap.Dispose();
    var img = iTextSharp.text.Image.GetInstance(Server.MapPath("~/Image.jpg"));
    img.SetAbsolutePosition(200, 400);
    PdfContentByte waterMark;
    using (MemoryStream stream = new MemoryStream())
    {
        PdfReader reader = new PdfReader(bytes);
        using (PdfStamper stamper = new PdfStamper(reader, stream))
        {
            int pages = reader.NumberOfPages;
            for (int i = 1; i <= pages; i++)
            {
                waterMark = stamper.GetUnderContent(i);
                waterMark.AddImage(img);
            }
        }
        bytes = stream.ToArray();
    }
    File.Delete(Server.MapPath("~/Image.jpg"));
    File.WriteAllBytes(sourceFilePath, bytes);
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    PdfStampInExistingFile("@aspsnippets.com")
End Sub

Private Sub PdfStampInExistingFile(ByVal text As String)
    Dim sourceFilePath As String = "C:\Users\anand\Desktop\Test.pdf"
    Dim bytes As Byte() = File.ReadAllBytes(sourceFilePath)
    Dim bitmap As Bitmap = New Bitmap(200, 30, System.Drawing.Imaging.PixelFormat.Format64bppArgb)
    Dim graphics As Graphics = graphics.FromImage(bitmap)
    graphics.Clear(Color.White)
    graphics.DrawString(text, New System.Drawing.Font("Arial", 12, FontStyle.Bold), New SolidBrush(Color.Red), New PointF(0.4F, 2.4F))
    bitmap.Save(Server.MapPath("~/Image.jpg"), ImageFormat.Jpeg)
    bitmap.Dispose()
    Dim img = iTextSharp.text.Image.GetInstance(Server.MapPath("~/Image.jpg"))
    img.SetAbsolutePosition(200, 400)
    Dim waterMark As PdfContentByte

    Using stream As MemoryStream = New MemoryStream()
        Dim reader As PdfReader = New PdfReader(bytes)
        Using stamper As PdfStamper = New PdfStamper(reader, stream)
            Dim pages As Integer = reader.NumberOfPages

            For i As Integer = 1 To pages
                waterMark = stamper.GetUnderContent(i)
                waterMark.AddImage(img)
            Next
        End Using

        bytes = stream.ToArray()
    End Using

    File.Delete(Server.MapPath("~/Image.jpg"))
    File.WriteAllBytes(sourceFilePath, bytes)
End Sub

Screenshot

Before

After