He kept the truck in two wheel drive and drove in second gear. The light of the unrisen moon before him spread out along the dark placard hills like scrimlights in a theatre.
I searched the word in a few dictionaries, and none of them included it as an adjective. ChatGPT says the word could not be used as an adjective. I could only assume that Cormac McCarthy meant the hills that resembled a placard sign (flat, maybe). Or has it got anything to do with the French 'placard' meaning a closet?
The quote is from the book "No Country for Old Men".