DTTG supports “open in app” functionality of ios. However, there are some third party apps that do not support it yet (e.g. PDF expert is one of them?). Maybe other users can comment here, which apps they use together with DTTG and its open in app functionality… the aim is to have a collection of apps that fully support the open in app feature (selecting a file within dttg and open it in another app, saving changes back to dttg)
You may want to read this thread.
I use Apple Pages (Word Processing) and Numbers (Spreadsheets)