PdfReader reader = new PdfReader(FilePDF);
 AcroFields af = reader.AcroFields;
            List<string> names = af.GetSignatureNames();

 PdfDictionary singleSignature = af.GetSignatureDictionary(name);
                PdfString asString1 = singleSignature.GetAsString(PdfName.CONTENTS);
                byte[] signatureBytes = asString1.GetOriginalBytes();

                RandomAccessFileOrArray safeFile = reader.SafeFile;

                PdfArray asArray = singleSignature.GetAsArray(PdfName.BYTERANGE);
                using (
                    Stream stream =
                        (Stream)
                        new RASInputStream(
                            new RandomAccessSourceFactory().CreateRanged(
                                safeFile.CreateSourceView(),
                                (IList<long>)asArray.AsLongArray())))
                {
                    using (MemoryStream ms = new MemoryStream((int)stream.Length))
                    {
                        stream.CopyTo(ms);
                        byte[] data = ms.GetBuffer();
 
                        ContentInfo contentInfo = new ContentInfo(data);  
                        SignedCms signedCms = new SignedCms(contentInfo, true);  
                        signedCms.Decode(signatureBytes); 
                        try
                        {
                            signedCms.CheckSignature(true); 
                        }
                        catch ( Exception ex)
                        {
                          ... ex.Message.ToString();
                         }


 и цикл по подписантам:
.. sinerInfo in signedCms.SignerInfos
... sinerInfo.Certificate - все данные из сертификата ...

...
..