Yes! Paperight is absolutely legal when used correctly. Photocopy shops who illegally photocopy large portions of books for their customers do this for their customers risk being shut down and fined if they are caught. Paperight provides photocopy shops with a legal alternative to legally copy, or print out and distribute books to their customers.
The books on Paperight have been put there with the permission of the rightsholder. You are allowed to download and print out these books, or make a legal copy from a traditional book, in exchange for a license fee. You then charge the customer for this license fee on top of your printing/copying and binding costs.
As long as you pay the license fee for every copy of a book that you print out or copy, you are operating legally. If, for example, you are printing out three copies of a book, you will need to pay the license fee three times. It is best to delete the download as soon as you have printed it to prevent an accidental re-print, and avoid breaking the law.