PM> Install-Package Spire.PDF

using Spire.Pdf;
using Spire.Pdf.Security;

namespace AddInvisibleSignature
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a PdfDocument object
            PdfDocument doc = new PdfDocument();

            //Load a sample PDF file
            doc.LoadFromFile("C:\\Users\\Administrator\\Desktop\\sample.pdf");

            //Load the certificate
            PdfCertificate cert = new PdfCertificate("C:\\Users\\Administrator\\Desktop\\MyCertificate.pfx", "e-iceblue");

            //Create a PdfSignature object
            PdfSignature signature = new PdfSignature(doc, doc.Pages[doc.Pages.Count - 1], cert, "MySignature");

            //Set the document permission to forbid changes but allow form fill
             signature.DocumentPermissions = PdfCertificationFlags.ForbidChanges Or PdfCertificationFlags.AllowFormFill

            //Save to another PDF file
            doc.SaveToFile("InvisibleSignature.pdf");
            doc.Close();
        }
    }
}

Imports Spire.Pdf
Imports Spire.Pdf.Security

Namespace AddInvisibleSignature
    Class Program
        Shared  Sub Main(ByVal args() As String)
            'Create a PdfDocument object
            Dim doc As PdfDocument =  New PdfDocument()

            'Load a sample PDF file
            doc.LoadFromFile("C:\\Users\\Administrator\\Desktop\\sample.pdf")

            'Load the certificate
            Dim cert As PdfCertificate =  New PdfCertificate("C:\\Users\\Administrator\\Desktop\\MyCertificate.pfx","e-iceblue")

            'Create a PdfSignature object
            Dim signature As PdfSignature =  New PdfSignature(doc,doc.Pages(doc.Pages.Count - 1),cert,"MySignature")

            'Set the document permission to forbid changes but allow form fill
             signature.DocumentPermissions = PdfCertificationFlags.ForbidChanges Or PdfCertificationFlags.AllowFormFill

            'Save to another PDF file
            doc.SaveToFile("InvisibleSignature.pdf")
            doc.Close()
        End Sub
    End Class
End Namespace

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

namespace AddVisibleSignature
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a PdfDocument object
            PdfDocument doc = new PdfDocument();

            //Load a sample PDF file
            doc.LoadFromFile("C:\\Users\\Administrator\\Desktop\\sample.pdf");

            //Load the certificate 
            PdfCertificate cert = new PdfCertificate("C:\\Users\\Administrator\\Desktop\\MyCertificate.pfx", "e-iceblue");

            //Create a PdfSignature object and specify its position and size 
            PdfSignature signature = new PdfSignature(doc, doc.Pages[doc.Pages.Count - 1], cert, "MySignature");
            RectangleF rectangleF = new RectangleF(doc.Pages[0].ActualSize.Width - 260 - 54, 200, 260, 110);
            signature.Bounds = rectangleF;
            signature.Certificated = true;

            //Set the graphics mode to ImageAndSignDetail
            signature.GraphicsMode = GraphicMode.SignImageAndSignDetail;

            //Set the signature content 
            signature.NameLabel = "Signer:";
            signature.Name = "Gary";
            signature.ContactInfoLabel = "Phone:";
            signature.ContactInfo = "0123456";
            signature.DateLabel = "Date:";
            signature.Date = DateTime.Now;
            signature.LocationInfoLabel = "Location:";
            signature.LocationInfo = "USA";
            signature.ReasonLabel = "Reason:";
            signature.Reason = "I am the author";
            signature.DistinguishedNameLabel = "DN:";
            signature.DistinguishedName = signature.Certificate.IssuerName.Name;

            //Set the signature image source
            signature.SignImageSource = PdfImage.FromFile("C:\\Users\\Administrator\\Desktop\\handwrittingSignature.png");

            //Set the signature font 
            signature.SignDetailsFont = new PdfTrueTypeFont(new Font("Arial Unicode MS", 12f, FontStyle.Regular));

            //Set the document permission to forbid changes but allow form fill
            signature.DocumentPermissions = PdfCertificationFlags.ForbidChanges | PdfCertificationFlags.AllowFormFill;

            //Save to file 
            doc.SaveToFile("VisiableSignature.pdf");
            doc.Close();
        }
    }
}

Imports System
Imports System.Drawing
Imports Spire.Pdf
Imports Spire.Pdf.Security
Imports Spire.Pdf.Graphics

Namespace AddVisibleSignature
    Class Program
        Shared  Sub Main(ByVal args() As String)
            'Create a PdfDocument object
            Dim doc As PdfDocument =  New PdfDocument()

            'Load a sample PDF file
            doc.LoadFromFile("C:\\Users\\Administrator\\Desktop\\sample.pdf")

            'Load the certificate
            Dim cert As PdfCertificate =  New PdfCertificate("C:\\Users\\Administrator\\Desktop\\MyCertificate.pfx","e-iceblue")

            'Create a PdfSignature object and specify its position and size
            Dim signature As PdfSignature =  New PdfSignature(doc,doc.Pages(doc.Pages.Count - 1),cert,"MySignature")
            Dim rectangleF As RectangleF =  New RectangleF(doc.Pages(0).ActualSize.Width - 260 - 54,200,260,110)
            signature.Bounds = rectangleF
            signature.Certificated = True

            'Set the graphics mode to ImageAndSignDetail
            signature.GraphicsMode = GraphicMode.SignImageAndSignDetail

            'Set the signature content
            signature.NameLabel = "Signer:"
            signature.Name = "Gary"
            signature.ContactInfoLabel = "Phone:"
            signature.ContactInfo = "0123456"
            signature.DateLabel = "Date:"
            signature.Date = DateTime.Now
            signature.LocationInfoLabel = "Location:"
            signature.LocationInfo = "USA"
            signature.ReasonLabel = "Reason:"
            signature.Reason = "I am the author"
            signature.DistinguishedNameLabel = "DN:"
            signature.DistinguishedName = signature.Certificate.IssuerName.Name

            'Set the signature image source
            signature.SignImageSource = PdfImage.FromFile("C:\\Users\\Administrator\\Desktop\\handwrittingSignature.png")

            'Set the signature font
            signature.SignDetailsFont = New PdfTrueTypeFont(New Font("Arial Unicode MS", 12f, FontStyle.Regular))

            'Set the document permission to forbid changes but allow form fill
             signature.DocumentPermissions = PdfCertificationFlags.ForbidChanges Or PdfCertificationFlags.AllowFormFill

            'Save to file
            doc.SaveToFile("VisiableSignature.pdf")
            doc.Close()
        End Sub
    End Class
End Namespace

using Spire.Pdf;
using Spire.Pdf.Widget;

namespace RemoveSignature
{
    class Program
    {
        static void Main(string[] args)
        {
            //Create a PdfDocument object
            PdfDocument doc = new PdfDocument("C:\\Users\\Administrator\\Desktop\\VisiableSignature.pdf");

            //Get form widgets from the document
            PdfFormWidget widgets = doc.Form as PdfFormWidget;

            //Loop through the widgets
            for (int i = 0; i < widgets.FieldsWidget.List.Count; i++)
            {
                //Get the specific widget
                PdfFieldWidget widget = widgets.FieldsWidget.List[i] as PdfFieldWidget;

                //Determine if the widget is a PdfSignatureFieldWidget
                if (widget is PdfSignatureFieldWidget)
                {
                    //Remove the widget
                    widgets.FieldsWidget.RemoveAt(i);
                }
            }

            //Save the document to another PDF file
            doc.SaveToFile("RemoveSignatures.pdf");
        }
    }
}

Imports Spire.Pdf
Imports Spire.Pdf.Widget

Namespace RemoveSignature
    Class Program
        Shared  Sub Main(ByVal args() As String)
            'Create a PdfDocument object
            Dim doc As PdfDocument =  New PdfDocument("C:\\Users\\Administrator\\Desktop\\VisiableSignature.pdf")

            'Get form widgets from the document
            Dim widgets As PdfFormWidget = DirectCast(doc.Form, PdfFormWidget)

            'Loop through the widgets
            Dim i As Integer
            For i = 0 To widgets.FieldsWidget.List.Count - 1 Step i + 1
                'Get the specific widget
                Dim widget As PdfFieldWidget = TryCast(widgets.FieldsWidget.List(i), PdfFieldWidget)

                'Determine if the widget is a PdfSignatureFieldWidget
                If TypeOf widget Is PdfSignatureFieldWidget Then
                    'Remove the widget
                    widgets.FieldsWidget.RemoveAt(i)
                End If
            Next

            'Save the document to another PDF file
            doc.SaveToFile("RemoveSignatures.pdf")
        End Sub
    End Class
End Namespace