Environment
GroupWise 2014 R2
GroupWise 18
GroupWise 18 Support Pack 1
GroupWise Mobility Service 2014 R2
GroupWise Mobility Service 18
Situation
You have successfully synced "forwarded as attachment" email type on your Apple device. Once you see such email on iOS based device, forwarded original email is displayed as EML attachment. Based upon a size of this email, an Apple device can auto-download it or you need to tap on it to manually download this item. After that you can view forwarded message. If that message had attachments, for instance a PDF file, opening the attachment does not show its contents.
Resolution
PLEASE NOTE: This problem is caused by a design change Apple made in 10.3.x versions and not a limitation of GMS. The same email downloaded with 10.1.x iOS version or earlier can open and display a PDF attachment from forwarded message.
The only condition when the attachment file can be opened is when original mail was "in-line forwarded". This is a possible workaround, use in-line forwarding instead of "forward as attachment" option.
There would need to be a significant design change at GMS site to "rewrite" Mime file of each such email into "in-line" format prior it gets converted into ActiveSync format and downloaded on a iOS device.