Gets the names of the preferences the viewer shall enforce, which the user cannot override (Enforce, PDF 2.0); empty when absent.
public IReadOnlyList<CosName> Enforce { get; }
ISO 32000-2 §12.2, Tables 147 and 148. The only name Table 148 defines is PrintScaling, valid only when
PrintScaling is given and not AppDefault. Other names are returned as written, with a diagnostic; elements
that are not names are dropped with a diagnostic.