Gets the catalog's Version entry, or null when it has none (or none that is a version).
public PdfVersion? CatalogVersion { get; }
ISO 32000-2 §7.7.2, Table 29 (PDF 1.4): it overrides the header when later, so an incremental update can raise the version without rewriting the header. Read from the catalog on every call. Version is the version the document conforms to.