The file identifier: the two byte strings of the trailer's ID entry.
public sealed class PdfFileIdentifier
ISO 32000-2 §14.4 and §7.5.5, Table 15. The first identifier is permanent, set when the file is first written; the second changes
with every revision. Each "shall have a minimum length of 16 bytes"; shorter ones (some producers write 8) are kept as they are,
with a FileIdentifierInvalid diagnostic. The ID is required in PDF 2.0 and in encrypted files, where it is never
encrypted. Read when FileIdentifier is called; the bytes are a snapshot.
kind:method, kind:property, kind:ctor. Press Esc to clear.ChangingGets the changing identifier, ID[1], based on the file's contents when it was last updated.PermanentGets the permanent identifier, ID[0], based on the file's contents when it was created.ToString()Returns both identifiers in hexadecimal, as a trailer writes them.