Got it working with MuPDF. =D
(just had some minor image artifacts)
To those interested, the command in Ubuntu 20.04 to get the program was:
$ sudo apt install mupdf
Then to run the PDF, I opened a terminal window where the PDF was placed and used the command:
$ mupdf [name of the file]
You can either put it in full screen (press f) or maximize the window (click in the square button in the top right of the window).
Either way, you'll need to expand the image as well. To do that, either press shift + w (fit to width) or shift + h (fit to height).
To get the page number, press shift + p, and it'll appear very faintly in the top left of the screen.
To go to a specific page, type the number of the page + g (e.g. 269g to go to the page 269).
There are a few more shortcuts I found, but I don't think they're needed for the game.