"Adobe XI" is not the name of an application. If you have Adobe Acrobat XI then follow the instructions provided. If you have Adobe Reader XI then you can't do it.
In that case you could use the trial version of Acrobat, or a separate tool, like this one I've developed recently: Batch Encrypt PDF Files
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Visit my custom-made PDF scripts website: http://try67.blogspot.com
Contact me personally: try6767@gmail.com