The Stanislaus County District Attorney also found no evidence of wrongdoing by students who obtained part of Palin's contract.
The former vice presidential candidate is scheduled to give a paid speech June 25 at the California State University, Stanislaus.
It is not known how much Palin will be compensated.
Campus president Hamid Shirvani had suggested that the contract was stolen from a recycling bin inside the office of a vice president.
Two students had claimed they found the documents in a campus dumpster.
Investigators say the two students declined to assist in the probe.