mupdf started simple but I think it also grew in a weird
direction (large codebase, some reflow I think, a strange epub
substandard support, etc .. all pretty useless to me and I'm not sure
how optional it it ... so no longer as lightweight as it could be ...)

mupdf-1.16.1-source/build/release$ du -sh platform resources source thirdparty
524K platform
42M resources
7.2M source
6.6M thirdparty

where resources is a directory with 155 fonts compressed and embedded into the exe.
Following (the quite old) pdf this  should be configurable
https://ghostscript.com/~robin/mupdf_explored.pdf
(ch. 18) 

I guess that mupdf's targets are windows/mac/Linux/Android but as a  base tool (eg ebook reader make senses on Android). 

License is AGPL
https://mupdf.com/license.html
like GPL plus
"""
Network Use – However, AGPL also has some conditions even if you are not distributing the software. For example, if you are using the software on your own company's equipment, but you are making the functionality of the software available to users interacting with it remotely through a computer network, and you make any change to the software, you must make the source code for your changed version available to users of the software. Take care to ensure that during network deployment that there is no code change that could invoke the source code availability obligation. This special requirement of AGPL is in Section 13; see the GNU web site for more details.

Bottom line, if you distribute our software, or make the functionality of the software available to users interacting with it remotely through a computer network, you must share your source code.
"""

and also the "Corresponding Source " subsection.


-- 
luigi