I just found out that there is a document browser called Zeal which is inspired by Dash for MacOS. You can save dozens of reference documents into your machine for offline browsing.

You can install to your ubunut machine by adding these ppas:

//QT5 dependencies && zeal ppa
sudo add-apt-repository ppa:ubuntu-sdk-team/ppa &&
sudo add-apt-repository ppa:jerzy-kozera/zeal-ppa &&
sudo apt-get update &&
sudo apt-get install zeal