ByteArrayOutputStream os = new ByteArrayOutputStream();
            byte[] b = bytesByFile(is);
            PdfReader reader = new PdfReader(b);
            PdfStamper stamper = PdfStamper.createSignature(reader, os, '\0');
            PdfSignatureAppearance appearance = stamper.getSignatureAppearance();
            appearance.setVisibleSignature(new com.itextpdf.text.Rectangle(36, 748, 144, 780), 1, signatureFieldNam);
            ExternalSignatureContainer external = new ExternalBlankSignatureContainer(PdfName.ADOBE_PPKLITE, PdfName.ADBE_PKCS7_DETACHED);
            MakeSignature.signExternalContainer(appearance, external,  8192);
            byte[] data = IOUtils.toByteArray(appearance.getRangeStream());
            return Base64.getEncoder().encodeToString(data);

function createSign(dataToSign, selectedCertID){
        
        // Получаю нужный сертификат
        var thumbprint = e.options[selectedCertID].value.split(" ").reverse().join("").replace(/\s/g, "").toUpperCase();
        try {
            var oStore = yield cadesplugin.CreateObjectAsync("CAdESCOM.Store");
            yield oStore.Open();

            var all_certs = yield oStore.Certificates;
            var oCerts = yield all_certs.Find(cadesplugin.CAPICOM_CERTIFICATE_FIND_SHA1_HASH, thumbprint);
            if ((yield oCerts.Count) == 0) {
              alert("Certificate not found");
              return;
            }
            var certificate = yield oCerts.Item(1);
        } catch (err) {
              alert('Certificate not found');
            return;
        }
        
        // Хэшируем. Используем CADESCOM_BASE64_TO_BINARY, поэтому оставляем пришедший base64
        try {
                var oHashedData = yield cadesplugin.CreateObjectAsync('CAdESCOM.HashedData');
                yield oHashedData.propset_Algorithm(cadesplugin.CADESCOM_HASH_ALGORITHM_CP_GOST_3411);
                yield oHashedData.propset_DataEncoding = cadesplugin.CADESCOM_BASE64_TO_BINARY;
                yield oHashedData.Hash(dataToSign);
            } catch (err) {
                throw "Failed to create CAdESCOM.HashedData: " + err.number;
            } 
         
         // подписчик
         try {
                var oSigner = yield cadesplugin.CreateObjectAsync("CAdESCOM.CPSigner");
                yield oSigner.propset_Certificate(certificate);
            } catch (err) {
                throw "Failed to create CAdESCOM.CPSigner: " + err.number;
            }

          // Подписываю хэш. Получаю base64 - отдаю его на сервер для дальнейшей вставки
            var oSignedData = yield cadesplugin.CreateObjectAsync("CAdESCOM.CadesSignedData");
            if (dataToSign) {
                try {
                   yield oSignedData.propset_DataEncoding = cadesplugin.CADESCOM_BASE64_TO_BINARY;
                   var Signature = yield oSignedData.SignHash(oHashedData, oSigner, cadesplugin.CADESCOM_CADES_BES);
                   return Signature;
                } catch (err) {
                    throw "Не удалось создать подпись из-за ошибки: " + cadesplugin.getLastError(err);
                }
            } 
}

public byte[] sign(FileId fileId, byte[] signature, String signatureFieldNam) {
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        try {
            byte[] file = getContent(fileId).orElseThrow(() -> new FileNotFoundException(fileId));
            PdfReader reader = new PdfReader(file);
            ExternalSignatureContainer container = new BrowserSignatureContainer(signature);
            MakeSignature.signDeferred(reader, signatureFieldNam, outputStream, container);
            return outputStream.toByteArray();
        } catch (IOException | GeneralSecurityException | DocumentException e) {
            log.error("while sign pdf file by external signature", e);
            throw new Exception("");
        } finally {
            try {
                outputStream.close();
            } catch (IOException e) {
                // Ignore
            }
        }
    }

 class BrowserSignatureContainer implements ExternalSignatureContainer {
        private byte[] sinedData;
        public BrowserSignatureContainer(byte[] signature) {
            this.sinedData = Base64.getDecoder().decode(signature);
        }
        public byte[] sign(InputStream is) {
            return sinedData;
        }
        public void modifySigningDictionary(PdfDictionary pdfDictionary) {
        }
    }