Hi,
I'm not sure about this, but Planetfall only comes with the manual and a map. Is this really all or should there be more documents that are missing in the GoG version and which documents do I need? I somehow get the impression that I need more information to get further in the game.

I dunno. I don't remember even reading the manual for the game when I played it (and finished it) on my C64 back in the day. However, here are scans of the original box material:

http://infocom.elsewhere.org/gallery/planetfall/planetfall.html

When you install Planetfall, under extras it lists what is supposed to be the Planetfall manual. But really, all of the extras are for Zork. In my opinion, this is sub-optimal.