How do I encrypt a file in Adobe reader XI?

If you are paying for it then you must have bought Adobe Acrobat DC not Acrobat Reader.

Acrobat Reader is only used to view pdfs and is free to use.

You can go to "Tools", then choose Protection -> Encrypt -> Encrypt with Password

If a dialog box appears asking for "Applying New Security Settings", click "Yes".

Choose "Acrobat X and later" (Recommended Option), if the recipient is using Acrobat X or Acrobat Reader X or later. Otherwise, select "Acrobat 7.0 and later" for better backwards compatibility.

* Ensure that "Encrypt all document contents" is selected.

* Ensure that "Require a password to open the document" is checked.

Type in a password in the "Document Open Password" field. Then click "OK".

Reenter the password you have chosen and click "OK". If a dialog prompts stating that the security settings will not be applied to the document until you save the document. Click "OK". Then, save the PDF document by clicking on "File" -> "Save". And verify that "(SECURED)" appears in the title bar.

Ref. http://help.adobe.com/en_US/acrobat/X/pro/using/WSD012A4E1-51D1-4bcd-BA9F-EF03C6F20BB6.html

Regards,

Rave