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.
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]
All content files are indexed and compressed in ZIM format, which makes them smaller, but leaves them easy to search and selectively decompress.
The ZIM files are then opened with Kiwix, which looks and behaves like a web browser, or with a suitably enabled conventional browser. Kiwix offers full text search, tabbed navigation, and the option to export articles to PDF and HTML.[8]
https://flathub.org/apps/org.kiwix.desktop
https://en.wikipedia.org/wiki/ZIM_(file_format)
Edit:
https://en.m.wikipedia.org/wiki/Kiwix#Description