Can .pdf files stored on SharePoint online can be opened using Acrobat Reader Mobile App?

Just like with OneDrive, you can store your PDFs in any of the supported cloud storage services. However,  if you're brwosing for. a PDF in a Micrososft Sharepoint directory and you tap on that file directly while on the Sharepoint cloud, it may not open natively in the Acrobat Reader mobile app.

This is the case with OneDrive, for example, where both, the Adobe app and the Microsoft cloud app are very likely not to integrate very well with one another.

If that is the case, then you will need to download and save the PDFs   locally to your mobile device first in order to be able to open them with the Acrobat Reader mobile app.