By lock, I'm assuming you mean 'secure'. If that's the case, you'll need Adobe Acrobat to secure your PDF files. PDF files created from FormsCentral do not have any security measures applied to them, nor is there a way of doing this from FormsCentral during the PDF-creation process. This Adobe TV tutorial covers securing PDF files in more detail: http://tv.adobe.com/watch/learn-acrobat-x/how-to-protect-pdf-documents/
Please let us know if you have any questions.
Thanks a lot.