We turn various online educational contents (such as Wikipedia, for example) into ZIM files, and these can be opened by Kiwix even if you have no connectivity whatsoever.
flathub.org/apps/org.kiwix.desktop
The ZIM file format is an open file format that stores website content for offline usage.[1] The format is defined by the openZIM project, which also supports an open-source ZIM reader called Kiwix. The format is primarily used to store the contents of Wikipedia and other Wikimedia projects, including articles, full-text search indices and auxiliary files.[2][3]