Search Digital Signatures in RTF File

using (Signature signature = new Signature("signed.pdf"))
{
    DigitalSearchOptions options = new DigitalSearchOptions()
    {
        // specify special search criteria
        Comments = "Test comment",
        // certificate issues criteria
        IssuerName = "John",
        // digital certificate subject
        SubjectName = "Test",
        // specify date range period of signature
        SignDateTimeFrom = DateTime.Now.AddMonths(-1),
        SignDateTimeTo = DateTime.Now,
        //
    };
    // search for signatures in document
    List signatures = signature.Search(options);
    Console.WriteLine("\nSource document contains following signatures.");
    foreach (var digitalSignature in signatures)
    {
        Console.WriteLine("Digital signature found from {0} with validation flag {1}. Certificate SN {2}",
            digitalSignature.SignTime, digitalSignature.IsValid, digitalSignature.Certificate?.SerialNumber);
    }
}