ProQuest is a large American information provider. ProQuest provides access to content used in teaching, learning and research, including scholarly journals, dissertations, ebooks, newspapers, periodicals, and historical collections.
ProQuest is not a publisher. Instead, it is what is known as an aggregator. Aggregators provide access to content created by publishers. Everything available on ProQuest has been published elsewhere, ProQuest collects material from different publishers and makes it available on one platform. You could probably find everything that's available on this platform on the public Internet, but with this database, you're searching one source as opposed to thousands.
As described in the Open Scholarly Resources section, traditionally, scholarly journals have been published by commercial publishers, who charge for access to the journals. More recently however, there has been a move towards making scholarly articles & journals freely available with no access restrictions.
Publicly Available Content Database brings together or links to full text for publicly available scholarly content from many different sources from around the world. It includes content from major subject repositories such as arXiv as well as open access journals. Content includes journal articles, pre-prints, books, conference papers and reports.
Subject coverage includes, but is not limited to:
Coverage of some publications dates back to 1970 and the database is updated daily. The database includes over 5 million documents from 3,500 titles and is international in scope, including titles from 81 countries.
For much of the content, the full text articles are presented within the database and are fully searchable. Where there is no full-text available in the database, there should be clear and reliable links direct to the external full text source.
A full title list is available here. Publications can also be browsed or searched through the Publications link on the home page.
Articles include all charts, tables, diagrams, and other graphical elements.