The public domain refers to works which are not protected by copyright. The copyright restrictions have sometimes expired or the creators have given up their claim through Creative Commons or alternative licensing. This is not synonymous with available on the internet.
Easy to copy does not mean free to copy.
January 1, 2019 is Public Domain Day Center for the Study of the Public Domain
Public Domain Frequently Asked Questions from Teaching Copyright
Public Domain Information Project Primarily focused on music.
The Internet Archive Millions of public domain books and documents. Great source of primary documents.
Project Gutenberg Thousands of public domain ebooks.