According to the PDF specification, it is possible to set the access and two passwords: the public password and the owner's password. If there are no passwords and everything is allowed to do with the document, then the document is not encrypted. If even one password is set or access is not allowed, then the document is encrypted.

 

The public password allows opening and viewing documents, and also some actions are allowed:

edit document;

copy text and graphics from the document;

add and change commentaries;

print document.

 

The owner password provides access to the document, including password changing and access permission.

 

If the owner's password is set, and the public password is not set, then, when opening a document, the password is not requested.