The KiCad software tool come with an additional component library package and a documentation package. Once you install all three packages you are set with a full fledged software tool for PCB design. Any necessary PDF documentation will be then available from the Help menu of each KiCad application (Eeschema, Cvpcb, etc.). The same documentation is available here too.
In this section you can find the official KiCad documentation in PDF directly pooled from the KiCad code repository.
Getting Started in KiCad. Essential and concise guide to mastering kicad for the successful development of sophisticated electronic printed circuit boards.
IDF manual This is not finished documentation, but is a good reference to understand how the IDF export functionality works.
Currently there is three doxygen docs available to the developers, these can be generated with make dev-docs, make doxygen-docs, and make doxygen-python in the kicad source. For convience they are also available on the following links and updaged by the Jenkins build server for each commit, so the ones here should always be the latest.
Documentation in other languages
A pretty good generic tutorial on how to use KiCad is made by Teho Labs.
In case you are interesting in scripting, please refer to the KiCad Scripting Reference Manual.
KiCad users mailing list
KiCad users can take advantage of the KiCad users mailing list for asking and answering questions. Before jumping in it please try to find the answers to your questions in the many reference manuals available.
A fun way to start your journey into PCB design with KiCad is to have a look at the many KiCad tutorial section.