Legal Question in Civil Litigation in California

You can attach what you like, and the opposing part can move to strike.

Usually there is no reason to attach anything to the complaint as the complaint and its attachments are not evidence; sometimes attachments can be used by the other side as a basis for a demurrer [you attach a contract that does not support the claims in the complaint, etc.].

I agree with Mr. Shers. Sometimes an attachment is required. For example, in a pleading based on breach of a written contract, either the terms of the contract must be set forth in the complaint verbatim, or a copy of the contract must be attached.

For the most part, there is no need for you to attach reams of documents to a complaint in a civil matter. The purpose of the complaint is to apprise the defendant and the court of the nature of your claims, not to attempt to prove your entire case at the outset.