/// <summary>
/// Changes the text field value.
/// </summary>
/// <param name="document">The PDF document.</param>
/// <param name="fieldFullName">Full name of the field.</param>
/// <param name="newValue">The new value of the field.</param>
/// <returns>
/// <b>true</b> - field value is changed successfully;
/// <b>false</b> - field value is NOT changed.
/// </returns>
public static bool ChangeTextFieldValue(
    Vintasoft.Imaging.Pdf.PdfDocument document,
    string fieldFullName,
    string newValue)
{
    // if PDF document has PDF interactive form
    if (document.InteractiveForm != null)
    {
        // find field by name
        Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormField field =
            document.InteractiveForm.FindField(fieldFullName);
        // if field is found
        if (field != null)
        {
            Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField textField =
                field as Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField;
            // if field is text field
            if (textField != null)
            {
                // set new value of the field
                textField.Value =
                    new Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextFieldStringValue(document, newValue);

                return true;
            }
        }
    }
    return false;
}

''' <summary>
''' Changes the text field value.
''' </summary>
''' <param name="document">The PDF document.</param>
''' <param name="fieldFullName">Full name of the field.</param>
''' <param name="newValue">The new value of the field.</param>
''' <returns>
''' <b>true</b> - field value is changed successfully;
''' <b>false</b> - field value is NOT changed.
''' </returns>
Public Shared Function ChangeTextFieldValue(document As Vintasoft.Imaging.Pdf.PdfDocument, fieldFullName As String, newValue As String) As Boolean
    ' if PDF document has PDF interactive form
    If document.InteractiveForm IsNot Nothing Then
        ' find field by name
        Dim field As Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormField = document.InteractiveForm.FindField(fieldFullName)
        ' if field is found
        If field IsNot Nothing Then
            Dim textField As Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField = TryCast(field, Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextField)
            ' if field is text field
            If textField IsNot Nothing Then
                ' set new value of the field
                textField.Value = New Vintasoft.Imaging.Pdf.Tree.InteractiveForms.PdfInteractiveFormTextFieldStringValue(document, newValue)

                Return True
            End If
        End If
    End If
    Return False
End Function