The foundation supports a bunch of other open source projects, after all there is a lot more to devices that run the Linux kernel then just the kernel.
Also, I found it a but funny that the foundation created the PDF using Adobe InDesign 19.4 (Windows), according to the metadata in file posted on their website. (original | archive of the PDF)
Whatever file format I use them in is also how I back them up, I backup my entire desktop’s and laptop’s data to an external hard drive and an online service provider. I’m sure a compressed format would be more space efficient but that would take much more time given my use case.
In the case of my laptop it runs Linux and the filesystem I use supports “transparent compression” (almost all contents of the drive are compressed with zstd), so I’m guessing any of the ROMs on there will have already been compressed as nuch as they can (but I’m not knowledgeable enough on the file format specs)