Gets or sets a value indicating whether objects the trailer does not reach are left out of the written file (garbage collection).
The default is false: every object the cross-reference information lists is written.
public bool RemoveUnreachableObjects { get; set; }
ISO 32000-2 §7.5 and §7.3.10. An object is reachable when a chain of indirect references leads to it from the trailer: from the
catalog (Root), the document information dictionary (Info) or any other trailer entry. A page removed from the page
tree, an annotation removed from its page's Annots, and everything only they referred to, are not written. An object
reached only as a stream's indirect Length is not written either; the stream gets a direct length (§7.3.8.2).
Objects the library does not interpret (JavaScript, XFA, 3D, rich media, multimedia) are kept like any other object when they are reachable, with their data unchanged. With Automatic, a dropped object's number becomes a free entry with its generation raised by one (§7.5.4).