Gets the default appearance string (DefaultAppearance) parsed, with its font resolved in the default resources;
null when the field has no default appearance string.
public PdfDefaultAppearance? ParsedDefaultAppearance { get; }
ISO 32000-2 §12.7.4.3. The font is looked up in the Font subdictionary of the nearest DR entry of the field or
an ancestor, then of the interactive form's DR (Table 224), which is where the clause requires it to be: Table 226
gives fields no DR, but some writers put one there and other readers honour it.
A string that departs from the clause (Problems) records DefaultAppearanceInvalid;
a font that no default resources hold records DefaultAppearanceFontMissing and leaves
Font null. Each read parses the string again and returns a new instance.