Show files My Book Live Duo on iPad

Would be great if anyone could help me with the following basic issue. Thanks in advance.

My father tries to view his files via his My Book Live Duo on his iPad at home. This works perfectly fine with respect to photos (via the WD Photos app), but he isn’t able to see other files (e.g. PDF, doc). What is the most convenient solution for viewing these type of files on an iPad? Is there an app that covers all file types?

Anyone?