Also when you swipe down the bookshelf, it will also trigger the library to look for any new information to sync from the server.
Once the update is completed, it will tell you what folders have new files available for downloading. You can touch this to also check if there is new content available.
You can then select to Download All files via the settings area within the app which will download the amount of data show. \
If you do not want to download everything at that time, go to the folders showing new content available and select individual file icons by touching on them for downloading. The document will automatically open once it is downloaded.