Gets or sets the changes the document permits after this signature, which makes it a certification signature; null
(the default) makes an approval signature. A certification signature shall be the document's first signature.
public PdfCertificationPermissions? Certification { get; set; }
ISO 32000-2 §12.8.1 (at most one certification signature; approval signatures follow it), §12.8.2.2 (DocMDP, Table 257) and
§12.8.6 (the catalog's Perms DocMDP entry, Table 263). PDF 1.5.