system icons i cant find

Hello,
i am changing some system icons but there are some i can’t find
pdf icon /png icon for example.
can someone tell me where they are?
they are not in the : /System/Library/CoreServices/Dock.app/Contents/Resources/

i am using el capitan

the main ones such as folders etc i have found and changed.
thanks for help ( maybe indicate me a tutoriel )

Due to Apple's new SIP (System Integrity Protection) inclusion in El Capitan many such files are hidden. The point is that Apple don't want people nosing around where they think they shouldn't. Changing System Icons is one of those areas. Even back in the dim dark ages when packages of custom icons were popular they still came with a warning for Mac users and the process of changing them was a cut and paste method using the Get Info windows. That however no longer works as it did requiring the use of Preview as well. Having said all that it is not for no reason that Apple have "forbade" us access to these files it is because there is the real possibility of damaging the file recognition system used by the OS. So much as you might like to customise your Mac I would advise against it, it may not cause any problems now but somewhere down the line when some other error occurs and you need to restore files or apps it just might.