Material that is in the Public Domain can be used for any purpose, including commercial, for-profit, and re-mixing.
Pride and Prejudice and Zombies is an example of a work in the Public Domain that was re-purposed and sold for profit.
The basic rule of thumb is that anything published before 1924 is in the Public Domain. However, as a rule of thumb, it does not perfectly apply to every item. (For example, some work-for-hire materials created before 1924 are still not in the Public Domain. Also, some works that were written prior to 1923, but not copyrighted until after 1923 are still not in the Public Domain.)
This chart from Cornell provides a breakdown of when different categories of materials become part of the Public Domain.