Offline documentation systems

[aviaryan]

Zeal is an offline desktop-based documentation viewer supporting many popular languages and frameworks. The docs are fully searchable by topic. http://zealdocs.org

DevDocs.io is a browser-based documentation engine with full offline support. It is faster than Zeal and recommended for web projects. http://devdocs.io

Both of these are cross platform and open source.

[singhpratyush]

Zealdocs is also available in official Ubuntu repository.
Link

[dufferzafar]

In case you're on a Mac, there's Dash.