In the Security tab, if you specify
Require password to open the file, the password will be required when a user opens the PDF file using any application that enforces passwords such as Adobe Reader, Adobe Acrobat Professional, or Apple Preview. If you specify
Require password to control printing, editing and security, the password will be required when a user opens the PDF file using an application that allows a user to modify privileges on the file such as Adobe Acrobat Professional. The password will be required before a user can modify the privileges on the file.