In order to verify that you are the intended recipient of the document, you need to be online to unlock the document. Internet connection is crucial in the initial unlock as the secured PDF or the secured web link (Web Viewer URL) will need to communicate back with our server to authenticate the User who's trying to access the secured contents. Our server will verify whether the User has access to your document or not; the permissions you've set; and it validates any other types of restrictions you may have applied to your document and/or User/group.
Once unlocked, some of your documents can be set up to be viewed offline for a time that has been predetermined. During offline access, you will not need an Internet connection to unlock your secured document (this is determined by the offline days set in the share settings - once the number of days in offline days setting expires, it will prompt the User to connect to the internet again to re-authenticate the document's security protocol).