Gets the default appearance string (DA): the field's or its nearest ancestor's, else the interactive form's; null
when none has one. Read as bytes, one character per byte; ParsedDefaultAppearance parses it.
public string? DefaultAppearance { get; }
ISO 32000-2 §12.7.4.3, Table 228 (required and inheritable for fields containing variable text) and §12.7.3, Table 224. A
text or choice field without one records DefaultAppearanceMissing.