Which files to version control for KiCAD?
When archiving a project that is a design based upon some specific CAD tool it is strongly suggested that you save all the produced collaterals (schematic PDFs, Gerber files etc). As designs age can be a problem to expect to be able to access and run the tools that created the collateral. 

This is particularly true in environments where a design may use these collaterals for production purposes over multiple decades. PDF files to read schematics and Gerber files to produce raw circuit boards have been in use for nearly 30 years now but on the other hand the CAD package after CAD package has come and gone. And even some that are still around in name may not run on modern platforms or will refuse to read old version files.