Is taking extra money considered usury (riba), and how can one dispose of it when the owners of the money are unknown, and is it permissible to give it as charity on their behalf?

This is not usury; rather, it is consuming people's wealth unjustly. You took an amount to photocopy the memorandum, and it was incumbent upon you to return the remaining amount to its owners and divide it among them. It is not permissible to give the memorandum to someone who has not paid, except with the permission of those who paid. You must repent and seek forgiveness, and search for the owners of the money to return their rights to them. If you do not find them, then give the money as charity on their behalf to the needy. If you find any of them later, you must return his share to him if he does not approve of the charity.