Many of the PDF files you create on a regular basis need some sort of protection—to prevent unauthorized access to, or manipulation of, your files. It isn’t necessary to secure every document you create, but for those you share or distribute, it’s a good idea.
Acrobat 9 offers several types of security. The option you choose for your document depends on the material involved, as well as on your intended audience:
Use passwords to prevent unauthorized access to documents. Restrict opening of your document with a password, and further restrict any types of changes with another password. You can also specify whether to allow users to copy, print, or extract content from a document.
Digitally sign your document, ...