For the secured web documents to work offline, a copy needs to be saved in the user's browser cache. This is a quick video shows how you can save a copy on your browser cache for offline use of the secured web document (Vitrium's Web Viewer):
Basically, the web document needs to be saved in the browser’s cache (as cookies) so it can be used later when the User decides to access it ‘offline.’ You’ll notice in the video also the delete button to remove this ‘saved copy’ from the browser for when their use is done.
We highly advise for the User to clean up their browser cache soon after they’re done reading the document offline as it will eventually fill up their browser caching storage (which gets stored on the hard drive). Simply put, if they save all the documents they have permission to for offline use, the browser cache can get quite large and it will start to slow down their computer speed for processing things.
Here’s an article online that you can read for more clarification on the browser cache: http://www.makeuseof.com/tag/browser-cache-makeuseof-explains/