W is my Galaxy S5 stock Email app unable to open docs from Hotmail account?

[AC Question]

Galaxy S5 stock Email app unable to open docs from Hotmail account

Hi I have a weird problem. My S5 running Android 5.0 can't open .pptx .pdf or .docx files from my Outlook Hotmail live account via the stock Email app. It makes no difference whether I choose "preview" or "download" - they won't open. When I use a 3rd party email app (blue mail) everything works fine.

Any idea what the issue might be.