The PubMed Central^® (PMC) database archives papers that fall under the public or open access policies of participating funding agencies and organizations. A current list of PMC-participating funding agencies in the U.S. is available at Funders and PMC.
For specific information on how to comply with the policies of these organizations, please see the funder's website. Information on on depositing a paper in PMC in compliance with a public access policy can be found at How Papers Get Into PMC.