Ah, the Chrome OS version.
This is AquaMail running inside Android (this part is provided by me) and this Android is running inside Chrome OS (this part is provided by Google).
So yes, there can be some rough edges and downright bugs in this "emulated" Android environment.
Please try "three dots" next to the attachment and choose "Save and open", maybe it'll work better (depending on what app on your Chromebook actually handles PDFs).