Hi come mettere una cifra,
If you'd like to password protect a PDF file, you need to use Acrobat. If you don't have Acrobat, you can try it for free for 30 days. Please see www.adobe.com/products/acrobat.html for more information.
In Acrboat, you choose File > Properties, and then click the Security tab to assign a document password.
Best,
Sara