public PdfInteractiveFormField GetFormField(
    string name
)

Public Function GetFormField(
    name As String
) As PdfInteractiveFormField

using (PdfDocumentProcessor pdfDocumentProcessor = new PdfDocumentProcessor())
{
    pdfDocumentProcessor.LoadDocument("FormDemo.pdf");
    ChangeFormFields(pdfDocumentProcessor);
    pdfDocumentProcessor.SaveDocument("FormDemo.pdf");
    Process.Start(new ProcessStartInfo("FormDemo.pdf")
       { UseShellExecute = true });
}

  private static void ChangeFormFields(PdfDocumentProcessor pdfDocumentProcessor)
  {
      PdfInteractiveForm acroForm = pdfDocumentProcessor.Document.AcroForm;

      //Change radio group field properties:
      PdfButtonFormField genderFormField = (PdfButtonFormField)acroForm.GetFormField("Gender");
      genderFormField.ToggleToOff = true;

      //Change text field properties:
      PdfTextFormField addressFormField = (PdfTextFormField)acroForm.GetFormField("Address");
      addressFormField.Multiline = false;
      addressFormField.InputType = PdfTextFieldInputType.PlainText;

      //Change combo box field properties:
      PdfChoiceFormField nationalityFormField = (PdfChoiceFormField)acroForm.GetFormField("Nationality");
      nationalityFormField.Editable = false;
      nationalityFormField.MultiSelect = false;
  }

Using pdfDocumentProcessor As New PdfDocumentProcessor()
  pdfDocumentProcessor.LoadDocument("FormDemo.pdf")
  ChangeFormFields(pdfDocumentProcessor)
  pdfDocumentProcessor.SaveDocument("FormDemo.pdf")
  Process.Start(New ProcessStartInfo("FormDemo.pdf") With {.UseShellExecute = True})
End Using

  private static void ChangeFormFields(PdfDocumentProcessor pdfDocumentProcessor)
    Dim acroForm As PdfInteractiveForm = pdfDocumentProcessor.Document.AcroForm

    'Change radio group field properties:
    Dim genderFormField As PdfButtonFormField = CType(acroForm.GetFormField("Gender"), PdfButtonFormField)
    genderFormField.ToggleToOff = True

    'Change text field properties:
    Dim addressFormField As PdfTextFormField = CType(acroForm.GetFormField("Address"), PdfTextFormField)
    addressFormField.Multiline = False
    addressFormField.InputType = PdfTextFieldInputType.PlainText

    'Change combo box field properties:
    Dim nationalityFormField As PdfChoiceFormField = CType(acroForm.GetFormField("Nationality"), PdfChoiceFormField)
    nationalityFormField.Editable = False
    nationalityFormField.MultiSelect = False
    End Sub