Okay, thanks, guys. The solution was as simple as opening up console and typing:
Code:
$ xdg-mime default mupdf.desktop application/pdf
Easy-peasy. I will confess, I'm kind of embarrassed it didn't occur to me that my OS hadn't assigned mupdf as the default PDF reader by--well, by default. I also feel bad that I didn't read the docs close enough to find out that the Calibre viewer didn't actually support that filetype.
Really, this isn't even a Calibre issue, so I feel sort of bad for posting on here, but, nonetheless, I really appreciate the help!