When adding another signature or upgrading the initial signature from PAdES B-T to PAdES B-LT:

byte[] addLTV(byte[] in)
            throws IOException, GeneralSecurityException, java.io.IOException {
        IOcspClient ocsp = new OcspClientBouncyCastle(new com.itextpdf.signatures.OCSPVerifier(null, null));
        ICrlClient crl = new CrlClientOnline();
        PdfReader reader = new PdfReader(new ByteArrayInputStream(in));
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        PdfWriter writer = new PdfWriter(out);
        PdfDocument pdfDoc = new PdfDocument(reader, writer, new StampingProperties().useAppendMode());
        LtvVerification v = new LtvVerification(pdfDoc);
        SignatureUtil signatureUtil = new SignatureUtil(pdfDoc);
        List<String> names   = signatureUtil.getSignatureNames();
        String sigName = names.get(names.size() - 1);
        PdfPKCS7 pkcs7 = signatureUtil.readSignatureData(sigName);
        if (pkcs7.isTsp()) {
            v.addVerification(sigName, ocsp, crl, LtvVerification.CertificateOption.WHOLE_CHAIN,
                              LtvVerification.Level.OCSP_CRL, LtvVerification.CertificateInclusion.NO);
        } else {
            for (String name : names) {
                v.addVerification(name, ocsp, crl, LtvVerification.CertificateOption.WHOLE_CHAIN,
                                  LtvVerification.Level.OCSP_CRL, LtvVerification.CertificateInclusion.YES);
            }
        }
        v.merge();
        pdfDoc.close();
        return out.toByteArray();
    }

the initial signature becomes invalid with an error “the document has been altered or corrupted since the Signature was applied.

Upon inspection, I found that the initial signature becomes invalid as soon as it’s read by PdfDocument or PdfSigner.

String srcFileName = "./BT.pdf";
        String outFileName = "./B-LT01.pdf";
        PdfDocument document = new PdfDocument(new PdfReader(srcFileName), new PdfWriter(outFileName), new StampingProperties().useAppendMode());
        document.close();

The code works fine with other PDF files. How can I resolve this issue?
my pdf file