How can I create PDFs which are not locked?

By default, the created PDFs are locked. To disable it, the setting is available in the server administration, in DOC settings:

Note: the server caches created PDFs. So if you have downloaded the PDF of a SIGN item and than change this settings, the setting will not have an effect until you remove the cached PDF. To clean the cache you can just rename the SIGN item (save and rename it back).