...without paying for it. It has worked on this computer for a long time, now it will not.
What has changed? Why am I being prompted to pay for something that has been working at no charge?
More information..... If I open a PDF from Explorer, I cannot password protect. If I open Acrobat DC, do a file...open... then select a PDF, I can password protect....
Compare the app which is running. Look under Help > About. My money is on you having a copy of Acrobat Reader (free) installed along with Acrobat Standard (definitely not free).