Here is C# code that demonstrates how to add long term validation (LTV) information to all digital signatures of PDF document: 
/// <summary>
/// Adds long term validation (LTV) information to all signatures of specified PDF document.
/// </summary>
/// <param name="inputPdfFilename">The filename of input PDF document.</param>
/// <param name="outputPdfFilename">The filename of output PDF document.</param>
public static bool AddVltInfo(string inputPdfFilename, string outputPdfFilename)
{
    // open PDF document
    using (Vintasoft.Imaging.Pdf.PdfDocument document =
        new Vintasoft.Imaging.Pdf.PdfDocument(inputPdfFilename))
    {                
        try
        {
            // add LTV info
            int count = Vintasoft.Imaging.Pdf.Tree.DigitalSignatures.PdfDocumentLtv.AddLtvInfo(document);
            if (count == 0)
                System.Console.WriteLine("LTV information is not required for this document.");
            else
                System.Console.WriteLine("LTV information is added.");
        }
        catch (System.Exception ex)
        {
            System.Console.WriteLine("Error: " + ex.Message);
            return false;
        }

        // save PDF document
        if (inputPdfFilename == outputPdfFilename)
            document.SaveChanges();
        else
            document.Save(outputPdfFilename);

        return true;
    }
}