From the About-page:

Did you know that you could store the entirety of Wikipedia on your phone? And read it anywhere, at anytime? [...]

We can make highly compressed copies of entire websites that each fit into a single (.zim) file. Zim files are small enough that they can be stored on users’ mobile phones, computers or small, inexpensive Hotspot.

Kiwix then acts like a regular browser, except that it reads these local copies. People with no or limited internet access can enjoy the same browsing experience as anyone else.

The software as well as the content are fully open-source and free to use and share.

My favorites for offline use are Wikipedia and iFixit, although the step-by-step photos are (by design) very low res.