Add annotations in android via alternate app+google drive?

There’s been some increase in demand for this possibility since last year, for the mobile and web viewers alike. One of such requests:

While not something we’re actively considering at the moment, the matter is on our radar and we continue tracking user demand.

As Stefan mentions on that thread above, if a PDF viewer uses ISO-conforming annotations, Paperpile should be able to automatically sync annotations in both direction from Google Drive.