I know that PDFs on a disc are classified under DVD-ROM content and can't be accessed through a menu item even if such a seelction exists (since it will just send you to a screen which says something like 'place the disc in a DVD-ROM drive on your computer and access the PDFs directly), so this is a minor exception to the 'features must be accessible through a menu selection' Rule.

However, what is the situation if the PDFs are on the disc but not mentioned anywhere on any menu, even to let you know they are present? Should they still go in as DVD-ROM content as usual or, because there isn't even a menu item to say "these are on the disc, have a look with a PC" should they not be entered as a Feature at all? Or do they count as hidden so add as an Easter Egg?

FWIW, in this case, they are mentioned on the cover, just not on the menus anywhere (so you have to check all the discs to find which one they are on!)

Why are you limiting your question to PDFs? There are other DVD-ROM features too that don't get mentioned in the menu. In fact I'd guess half of my discs with DVD-ROM content don't make any mention of it at all. We never handled these as easter eggs so why should we with PDFs? For me a dvd easter egg is something I can watch with my dvd player.