Acrobat must be run from an interactive task and not on a server, so it is not suitable for use from an automation scheduler.
Thanks for your prompt response.
The problem with automation that you're trying to set up is that in this case your machine would serve as a server and Acrobat license does not permit that. Actually, you can launch Acrobat from a terminal/command window with some switches that perform some functionality, like opening a specific PDF, but encryption is not on the list of these switches nor any other meaningful functionality. I think that this is because that would open the door to use this command window as a server.