What kind of paper did monks used?

By the 12th century, parchment and paper were in use. Very early they would have used products like parchment and vellum.

Parchment is a writing material made from specially prepared untanned skins of animals-primarily sheep, calves, and goats. It has been used as a writing medium for over two millennia. Vellum is a finer quality parchment made from the skins of young animals such as lambs and young calves.