Looks like a Microfilm reader?
As for the joke...
It's like the dungeon was so huge they had to "shrink" it to microfilm to be able to fit in "1 page".
I wouldn't say it's a great joke, but that's what I got out of that.
The things in the boxes are microfiche, an archival solution used before scanners were common. The images on them are fingernail-sized and require a bulky device (also pictured) to read.
I work with these things. A fiche is the size of my hand and can hold about 10x23 A4 pages, maybe 10x24 if very dense. Or one ginormous dungeon map, if you really wanted.
Apparently the maximum limit of a PDF page is 15 million inches in each direction. That's roughly 381 km or 236.7 miles. This is larger than The Netherlands in terms of square km or miles. If you really want to have a mega dungeon, there's your limit ;)
Of course, this cannot be printed, but a page is a page!