Gets the embedded file stream (EF, by key preference UF, F, Unix, Mac, DOS), or null.
public PdfEmbeddedFile? EmbeddedFile { get; }
ISO 32000-2 §7.11.3, Table 43 (PDF 1.3), and §7.11.4. When EF is present but none of its entries is a stream, the
specification is still readable; the embedded file reads as absent with an EmbeddedFileMissing diagnostic.