The primary Endless OS desktop user documentation is available within Endless OS itself. This includes:
Within Endless OS, all such content is available offline.
The help center content is also available to be viewed online.
Click on your user icon in the bottom right of your screen (shown below with a "D").
On the menu that appears, click Help.
The Help app will now launch.
Alternatively, you can find the Help app on your desktop, or by using the desktop search bar to locate the Help app, or by searching for content included in the help app.