The Art of Riven pdf that comes with the Deluxe Edition contains a strange "Easter Egg". On page 73, there is a magenta rectangle. It looks like it's hiding something. So, I opened it in Acrobat Professional and deleted the offending polygon to reveal more concept art. Was Cyan really trying to hide this? Seems like a major oversight if they thought no one would be able to manipulate a pdf document. Or, did they do this as a sort of Easter egg?

Very interesting! Thank you for sharing and including the image.

I am personally holding off buying the artbook just yet until I am a few playthroughs deep. I will need the artbook to fill the void then. So I cannot speak for the full DLC. Are there more instances like this in the PDF? How many pages are there?
From the image you attached it seems to me maybe they didn't have image credits for that reference photo (yet) and hid it until the issue is sorted out, then forgot about it in the turmoil of launch. Perhaps.
I cannot imagine the photo to be a genuine Easter egg. I also cannot imagine they would implement Easter eggs in such a way (PDF "tricks").

Anyway. Interesting find!

I have reached out to Cyan and they confirmed it is an error, not an easter egg. They said it will be "addressed in an update." So, I imagine it'll likely be fixed when the next update for the game itself happens, is my assumption. I'll report here again when it is fixed, unless someone else beats me to it.

Not meaning to double post but they have updated the Art Book. Curiously, all they did was remove the pink rectangle entirely but without restoring the image that was behind it. Don't know if that is also a mistake or if the reason the pink rectangle was there to begin with was to purposefully block the image all along. Regardless, at least now it doesn't stick out like a sore thumb with the pink.

EDIT 1: I have been informed it was removed because, "The image was a reference image, not a concept. That's why it was hidden in the first place. Cyan doesn't have the rights to redistribute reference images, so it was intentionally removed." So, that explains it. For whatever reason the "cover up" was pink (likely by mistake) instead of white to seamlessly cover it.

EDIT 2: A fan did a reverse image search and found the actual image. [I would link to it but the forum isn't letting me for some reason, so attached in the image of the webpage.] So there really is no reason to keep the old version of the Art Book in any form since it wasn't an image made for the game.