I don't recall leaving them out other than to dissipate the smell of Ammonia the paper had a yellow coating and needed to be kept in the dark the rolls arrived in thick black plastic covering.
The machine we used UV light to expose the paper which was in contact with the original once exposed it was developed by the ammonia the sheets were very large A0 or perhaps bigger.
This mind you is memories from 45 years ago
They were then folded down to A4 size I can't remember exactly how they were folded but it left them like a pouch so they didn't unfold and stayed flat.
Cheers
Mike T