Starting a week ago, I am no longer able to open pdf files on either iPhone or iPad through files of Microsoft One Drive or Files. I am only able to open them within Adobe Acrobat. All IOS systems are updated and the problem continues. "The file cannot be displayed. Sorry there was a problem opening the viewer" is the exact error. I am able to open Adobe Acrobat and the same file opens just fine. Any clues.
If the files are opening fine with the Adobe Reader mobile app, then there's really not a problem with the Adobe mobile app.
Your inquiry is directly asking about a specific feature of the mobile device that you own and a Microsoft cloud service integration.
You should consult guidance with Apple and Microsoft directly.
Other than that, is to consider the fact that sometime iOS and macOS , to include Micrososft family of newer operating aystems, do have a tendency of reverting back to their own default PDF handlers/viewers.
If that would be the case, I would recommend to check if there is a way for you to reset preferences and make Adobe Reader app the default PDF viewer.
Since the file viewer in iOS is also a browser, you may want to check if this could be related to how the PDF are trying to open by default: on the web browser or downloaded and then automatically opening in a default PDF handler other than the Adobe Reader.