I have an interesting issue trying to get files which have been attached to calendar items to view on a device which isn't always online.
Office based worker (using Outlook on a Windows PC) creates a Word document with details of a job for Engineer. The Engineer has an iPad and is using the standard iOS Mail App. The iPad only has Wifi. When connected to WiFi calendar sync works as expected and the attached Word document can be opened.
When the Engineer is away from the office, without wifi, and goes back to try and open the Word document again it may or may not open. Sometimes the files shows as "0.0kb" size, sometimes it shows the correct size, but selecting it resulting in a page showing the document name only.
The other interesting thing is that once this has happened the document can't be opened even when the device is back on Wifi. The only way to get the attachment to open is to to ask someone to access the calendar item via Outlook on a Windows PC, make a minor change to it, and then wait for it to sync - THEN the file opens again.
I've tried this on my own Office 365 tenant and the customers and I can recreate the problem with my own iPhone by setting it to air-plane mode.
I've tried the Outlook app for iOS however it doesn't appear to handle calendar attachments at all unfortunately.