@mac_heibu I had no idea... but as @garrettm30 pointed out, they're allowed in Macs.
@garrettm30 I think it was more the fact that some of the linked files, PNG images, were approaching 30MB. When I resized those original image files via Affinity Photo, and got them down to 5MB, it worked. But I couldn't replace them in Affinity Publisher without it crashing after 10 seconds, due to the multiple huge files I had linked. So I think I opened it in Affinity Photo and replaced them there. Doing so made it far more manageable for Affinity Publisher to handle the file.
Also, as @GabrielM pointed out, I should have more HD space available than the 10GB I was working with at the time. Before, it was eating up all that space as it processed the save, all the way to having no space left and I'd have to force quit and restart. Since I got my free space up to 30GB, that hasn't been an issue.