Well, generally it is published for researchers visiting the Archives Centre, so they can prepare in advance and save time (it's not unusual to wait 30/45 minutes for any document you request so it's better to ask as much as possible as soon as you arrive).
It depends on the departement, but they generally have a reproduction service, which can make a copy for a price but it could be expensive if you want many pages (it's done professionally, not with a family camera). And they won't do any search for you if the document you want is part of many (say a single marriage contract from a batch of notary papers). They can also refuse to photocopy hardback books or fragile papers to not damage them.
My advice would be to ask a favour from someone visiting the archives, it certainly is the simplest way, but again it depends on the type of document you're looking for.