I have three email addresses on new Blackberry Classic. Only one of the emails I cannot view some of the attachments. If I forward complete email to one of the other addresses, the document opens.
If I delete email from settings, and set up again, the attachments will open for awhile and then it will stop after a few days.
This was happening on my old Blackberry Z10 and now on the Classic.
I will suggest you to check with Blackberry support regarding this.