Linux Command Library is a Android app that works as a manual for Linux commands, something like tl;dr or manpages but for phone with GUI and can directly search for categories such package manager tools, ssh, git, etc.
The neat part of it is that you don’t have to guess a command, for example, if I want to descompress something (and I don’t have internet connection) and I don’t know the command I’m pretty much fucked, I can use the manpages but first I need to know the command to use it.
Maybe not exactly what you are looking for, but you could use a website archiving solution to store one of those online man-page collections. I know it sounds like overkill, but it’s actually easy to setup and can be used for all kinds of other things as well. For instance, I have a local docker container running SOSSE for C++ and Python references and for all kind of libraries and APIs I use for software development. This way I have everything I need on my laptop and can work anywhere, even if there is no network connection.