Pdf files cannot be opened in iOS-App

For some time now pdf files can no longer be opened in iOS App . Clicking on it will not open the file, but it will select to share the file.
I can’t open the pdf. I have to export it to another app to be able to read it, that’s pretty tedious.
All other files I can open normally. Only PDF not.
iOS 13.5.1, App, NC 18.0.5
Thanks for your help!
Kind regards

On Android i also must share to a pdf-app.
Perhaps the pdf-viewer is only in the web interface.