Got Asus Chromebook Flip C302 as well, and having same issue. Can't open PDF documents as attachments which is beyond annoying. Has to be a Chromebook Issue, as I can do it no Problem on my phone, but not on the Chromebook. Only work around is the Web, however need offline support. Any using a Pixelbook with this issue? Considering upgrading to have the best of the best and also for Business, as this will be my only device. Would great appreciate any work arounds or resolution to this issue. I contacted support, and followed their steps to uninstall app, restart, and install again and still not working for me. Beyond frustrating.