The default .pdf viewer in your Linux distro should be enough.. on Debian Trixie with GNOME it seems to be Evince and it works well and lightweight (especially compared to anything Adobe related!). I don't know too much about Termux but I guess if you can install Evince in there then it should be fine.
Otherwise like others mentioned most web browsers nowadays can view .pdf files, people usually have a web browser already installed so that's probably the easiest solution unless you need/want something different.