Every package in Latex comes with its documentation. Hence, most LaTeX-distributions contain the “complete” documentation by default, which is accessible without an internet connection.
In order to access a documentation file of any package, go to the command line and type:
texdoc package-name
For example, to open the documentation of the hyperref package, use:
texdoc hyperref
The texdoc command opens the documentation in the default PDF viewer, depending on your OS and configuration.