Problems opening files in emails

I wasn't sure how to search for this, so I apologize if this has been asked and answered.
Personally, I use the apps and don't have this problem, but others at work do. I think it's primarily (always) when opening a Word or Excel file in an email while using Outlook online. They'll get an error message or out of space message when trying to open it. They can download it, save it, then open it, but not just open it directly from the email.

We're phasing Office 2016 out and going to 365 in January, but in the meantime some that have the above issue have 2016 installed, but some don't...so they're just using 365 online.

Hoping there's a simple answer I'm just not thinking about.