The first question a user of any copyrighted work should determine is this: Is the work covered by copyright?
An easier question to answer is this: Is the work in the Public Domain?
Since 2001, it has been possible for authors to give blanket permission for certain uses of their work by using a Creative Commons license. For example, if the author were permitting you to use the work in any way you like as long as you acknowledge him or her as the author, he/she might choose a CC-by license:
Other licenses and their restrictions may be found at Creative Commons. Most Open Access publications are published under a Creative Commons license.
Works licensed by Creative Commons must be used only with the restrictions (if any) that the CC license places on the work. Uses beyond those licenses may require you to either seek permission or to apply Fair Use.