Gets the revisions of the file, oldest first: the original file, then one per incremental update.
public IReadOnlyList<PdfRevision> Revisions { get; }
ISO 32000-2 §7.5.6. The document itself always shows the newest revision: for every object number the most recent cross-reference section that lists it decides, and a number it marks free is deleted even when an older section has the object. A linearized file without updates has one revision. An incremental save adds the revision it wrote (IncrementalUpdate).