Introduction

public static void SignAndValidate()
{
  Console.WriteLine( "=== SIGN AND VALIDATE ===" );

  var outputFileName = "Sign.pdf";
  var outputPath = SignaturesSample.SignaturesSampleOutputDirectory + outputFileName;

  // Create a pdf document.
  using( var pdfoutput = PdfDocument.Create( outputPath ) )
  {
    // Get the first page of the document.
    var page = pdfoutput.Pages.First();

    // Add title.
    var titleStyle = TextStyle.WithFont( pdfoutput.Fonts.GetStandardFont( StandardFontType.Helvetica ), 15d );
    page.AddParagraph( "Sign a document", titleStyle, new ParagraphStyle( ParagraphHorizontalAlignment.Center ) );

    // Add a NumberFormField.
    page.AddText( "Number:", new Point( 50, 130 ) );
    var numberField = new NumberFormField( 32.66d, new Rectangle( 125, 122, 120, 22 ) );
    numberField.Border.Color = Color.Green;
    numberField.Border.Width = 1;
    page.FormFields.Add( numberField );

    // Add a CheckBox
    page.AddText( "CheckBox:", new Point( 50, 190 ) );
    var checkBoxField = new CheckBoxFormField( true, new Rectangle( 125, 182, 20, 20 ) )
    {
      TextStyle = TextStyle.WithFontAndBrush( pdfoutput.Fonts.GetStandardFont( StandardFontType.CourierBoldOblique ), 12d, Brushes.Red )
    };
    page.FormFields.Add( checkBoxField );

    // Add a SignatureFormField.
    page.AddText( "Signature:", new Point( 50, 250 ) );
    var signatureField = new SignatureFormField( new Rectangle( 125, 242, 120, 22 ) )
    {
      Border = new Border() { Color = Color.Blue, Width = 2 }
    };
    page.FormFields.Add( signatureField );

    // Create a credential from a Pfx file using password:12345678 and a SHA-512 Digest algorithm.
    var credential = new PfxSigningCredential( SignaturesSampleResourceDirectory + "TestSignature.pfx", "12345678", DigestAlgorithm.Sha512 );

    // Create Signature options to use when signing the document.
    var signatureOptions = new SignatureOptions()
    {
      Date = DateTime.Now,
      PhysicalLocation = "Montreal, Canada",
      Reason = "Authenticate important document"
    };

    // Sign the document from the signatureField, with the pfx credential and the defined options.
    // No need to save the document when signing, it is automatically saved.
    // Modifying pdfoutput.Pages[ 0 ] after signing will invalidate the signing.
    pdfoutput.Sign( signatureField, credential, signatureOptions );

    Console.WriteLine( $"Info exported to path: {outputFileName}" );
  }

  // Reload the document to validate it's signature.
  using( var doc = PdfDocument.Load( outputPath ) )
  {
    // Validate all the signature for the document's SignatureFormFields.
    var result = doc.ValidateSignatures();
    // Make sure the first Signature is valid.
    if( result[ 0 ].CertificateTrusted )
    {
      Debug.Assert( SignatureValidationStatus.ValidAndCoversWholeDocument == result[ 0 ].Status );
    }
    else
    {
      Debug.Assert( SignatureValidationStatus.ValidButUntrusted == result[ 0 ].Status );
    }
  }
}