There are 2 conditions for a work to be protected under copyright:
- originated from the author
- involves the author's skill and labor
(Note that "original" does not necessarily mean "creative". Also, the standard of "original" varies greatly between countries.)
Under these conditions, it is fair to say that a legal document, such as a contract or license agreement, should be protected by copyright.
Copyright, however, protects only the expression of the idea but not the idea itself. That is, while a document is protected under copyright, you can draft another document with different wordings which expresses the same idea.
Under the Merger doctrine, if there are only very few ways to express an idea, then the expression enjoys no copyright. This is because if that expression is protected, the protection would naturally extend to the idea itself. Therefore, common clauses in a contract or agreement are unlikely to be protected by copyright.
If a legal document is sufficiently creative, it may also be protected under copyright. In American Family Life Insurance Co. of Columbus v. Assurant, Inc., American Family Life Insurance Co. of Columbus (AFLAC, plaintiff) has spent significant resources to draft an insurance policy which customers find easier to read. AFLAC sued the plaintiff (Assurant, Inc.) for copying statements in their policy.
The court asserted that the narrative nature of AFLAC's policy was sufficiently different from "the words and expressions [that] are commonplace within the insurance field", thus entitled to protection as copyrighted work.
Conclusion: contracts and agreements can be protected by copyright. However, it is unlikely that one which only contains common clauses is protected. Nevertheless, copying an entire document may not be a wise idea. If you have doubts in drafting one, you should consult a lawyer.