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This is the most common way of locating open access materials. As shown in the example below there will often be two links, one to the freely-available PDF version in an open access repository and the other to the published version, access to which will be dependent on a subscription:
The following are specific open access search engines:
SocArXiv, open archive of the social sciences, provides a free, non-profit, open access platform for social scientists to upload working papers, preprints, and published papers, with the option to link data and code.
bioRxiv is a free online archive and distribution service for unpublished preprints in the life sciences. It is operated by Cold Spring Harbor Laboratory, a not-for-profit research and educational institution.