Open Access Resources are research materials have been made available to the general public, free of charge: data and datasets, books and articles, including scholarly research articles. This is not the same thing as public domain works, or those items that have fallen out of copyright protection; open access authors have chosen to share freely, so anyone can use the works legally without permission or fees.
PMC is a free full-text archive of biomedical and life sciences journal literature at the U.S. National Institutes of Health's National Library of Medicine (NIH/NLM).
HathiTrust is a partnership of major research institutions and libraries working to ensure that the cultural record is preserved and accessible well into the future. The HathiTrust Digital Library includes millions of digitized book titles, most of which were published before 1923.
Public Library of Science, 2010, https://flic.kr/p/8L4egi