Works that are in the Public Domain can be copied with no need to obtain permission or pay royalties. A work may be part of the Public Domain for one of the following reasons:
Resources dealing with Public Domain:
"Open access" works "are digital, online, free of charge, and free of most copyright and licensing restrictions" (Suber, 2004-2015). The author of an Open Access work retains copyright and determines how their resources can be used. For example, some authors allow derivatives of their work while others do not. To learn more about Open Access, read Open Access Overview by Peter Suber.
A Creative Commons license is a common indicator used with open access works that describes the copyright owner's conditions for use of their work.
You can search for Open Access publications using the following tools: