Gets the page mode used on leaving full-screen mode (NonFullScreenPageMode): UseNone (the
default), UseOutlines, UseThumbs or UseOC.
public PdfPageMode NonFullScreenPageMode { get; }
ISO 32000-2 §12.2, Table 147. Meaningful only when the catalog's page mode is FullScreen; it "shall be ignored otherwise", which is the caller's to apply: the value is returned whatever the page mode.