https://www.ncbi.nlm.nih.gov/pmc/

"PubMed Central® (PMC) is a free archive of biomedical and life sciences journal literature at the U.S. National Institutes of Health's National Library of Medicine (NIH/NLM). In keeping with NLM’s legislative mandate to collect and preserve the biomedical literature, PMC serves as a digital counterpart to NLM’s extensive print journal collection. Launched in February 2000, PMC was developed and is managed by NLM’s National Center for Biotechnology Information (NCBI). https://www.ncbi.nlm.nih.gov/pmc/

"Free Access: A Core Principle of PMC

As an archive, PMC is designed to provide permanent access to all of its content, even as technology evolves and current digital literature formats potentially become obsolete. NLM believes that the best way to ensure the accessibility and viability of digital material over time is through consistent and active use of the archive. For this reason, free access to all of its journal literature is a core principle of PMC.

Please note, however, that free access does not mean that there is no copyright protection. As described on our copyright page publishers and individual authors continue to hold copyright on the material in PMC and users must abide by the terms defined by the copyright holder.

"How Journal Articles are Provided to PMC

PMC is a repository for journal literature deposited by participating journals, as well as for author manuscripts that have been submitted in compliance with the public access policies of participating research funding agencies. PMC is not a publisher and does not publish journal articles itself.

PMC offers publishers a number of ways in which to participate and deposit journal content in the archive. Journals that would like to participate in PMC must meet PMC’s minimum requirements, submit a formal application, and undergo a review of the scientific and editorial quality of the content of the journal as well as a review of the technical quality of their digital files. More information on requirements for PMC participation and the review steps is available at Add a Journal to PMC and in the FAQ.

"PMC’s Integration with other Resources

In addition to its role as an archive, the value of PMC lies in its capacity to store and cross-reference data from diverse sources using a common format within a single repository. With PMC, a user can quickly search the entire collection of full-text articles and locate all relevant material. PMC also allows for the integration of its literature with a variety of other information resources that can enhance the research and knowledge fields of scientists, clinicians and others.

"International Collaboration and Durability

NLM is collaborating internationally with other agencies that share the goals of PMC. Maintaining copies of PMC’s literature in other reliable international archives that operate on the same principles provides greater protection against damage or loss of the material. At the same time, the diversity of sites allows for the possibility of more and even greater innovation, ensuring the permanence of PMC over the long-term." (https://www.ncbi.nlm.nih.gov/pmc/about/intro/,   accessed 12-2016).