Cannot combine files due to files being password-protected

Help please! I get an error message when trying to combine several files that I created. It says that they cannot be combined bc they are password-protected. How can I remove the passwords? Also how can I adjust my settings so that they don't get password-protected in the future when I create them. I am using Adobe Acrobat DC on a MacBook Pro running OS X Yosemite version 10.10.5. Thanks for any suggestions!