Yes! Public domain works are not copyright protected. Some works such as those from the Federal government and some state government works never were copyright protected.and are in the public domain. Some copyright holders give up their copyright and their materials are in the public domain. Very old works for which the copyright has expired after a very long time are in the public domain. Click on public domain chart to see University of North Carolina site for details on expiration of copyrights.
You have no copyright problem if you are using your own materials that you have created and for which you have retained your rights as the copyright holder. If you have assigned your copyright to a publisher or other entity without preserving your right to use the materials, you don't control use of those materials anymore.
The creator of an original work can decide that they want to share a work without any strings attached. They may waiver their rights and explicitly designate that the work is in the public domain. They no longer have any control of the work. You can use works designated public domain in any way you wish without asking permission. Also, check out Creative Commons that is in the next section.