No, you can access your titles on ProView while you are offline. You will need to first download your titles for offline use, which requires an internet connection.
You can download publications from the library by choosing "Download title" from the overflow menu on the title card of the publication you wish to download. You can also download a publication from within an opened title by clicking the download icon in the header. Once the titles have been downloaded, you can access them through the ProView web application without an internet connection. You can access, read and search your downloaded content while offline.