tl;dr: The only change, aside from the increased number of broken bookmarks, is that the misleading Fig. 4a in the page 7 has been splitted into Fig. 4a and 4b with different axes.
What's up with "Error! Bookmark not defined"? Is this some kind of trolling? I guess its presence in the first version is understandable if it was posted in hurry, but why leave it in the revised version?
This can happen when saving a Word document as PDF. You don’t see it in the Word document before, but it ends up that way in the PDF. They apparently didn’t double-check the PDF.