Gets or sets other certificates that may complete a chain the signature does not carry in full (intermediates from a document security store or a local cache). They are not trusted. Empty by default.
public IReadOnlyList<X509Certificate2> ExtraCertificates { get; set; }
ISO 32000-2 §12.8.3.3.1 (the chain in the CMS object) and §12.8.4.3 (the document security store).