People are often asked to obtain a certified copy of documents such as passports, transcripts, birth certificates, translations, etc. In California, the law only allows a Notary Public to certify copies of his/her journals and Power of Attorney Documents (CA Government Code sections 8205(a)(4), 8205(b)(1), and 8206(e)).
It is absolutely illegal for a California Notary to certify a copy of anything other than the aforementioned documents.
If you need an official copy of any document, it is best to go to the source and inquire about obtaining certified or official copies. If you do not need an official copy, but are still being asked to have it certified, a California Notary will not be able certify the copy except for the two examples mentioned above (journals and powers of attorney).
A notary can, however, notarize the signature of an individual on a statement or letter indicating a copy is genuine. Please be aware that this does not mean the notary is certifying the copy - only the identity of the individual signing the document. It is the individual signing the document that is responsible for the document's content.
In some cases, people choose to use a form called "Copy Certification by Document Custodian" as a substitute for a certified copy from the original source. Please be aware that there is no guarantee that the document will be acceptable to the entity requesting the copy certification. You should always ask the entity that is requesting the certification if this solution will work for them.