I think I have figured out the problem.The issue lies here.

  public void modifySigningDictionary(PdfDictionary pdfDictionary) {
    pdfDictionary.put(PdfName.FILTER, PdfName.ADOBE_PPKLITE);
    //注意这里
    pdfDictionary.put(PdfName.SUBFILTER, PdfName.ADBE_PKCS7_DETACHED);
}