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The PDF viewer app shows the file contain within the Nextcloud screen so you should be able to see if it’s activated/used or not.
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The Android Nextcloud app aim is file synchronization. For all other function it relies on the Android app environment. As always it is better to focus on the main competency than blowing-up the app by trying to implement functions that other apps are better in. Therefore you will always need other 3rd-party apps to show e.g. PDF files, sync contacts and calendars or read Nextcloud news etc.