To my knowledge, there is no way to set this to a default preference for users from the service side. This also depends on which service you use. Word Online, for example, converts to PDF use page layout settings and then sends to your local print drive which can be configured. Excel Online though - has an internal dialog that defaults to A4 (and you can't change it).
For office applications on the desktop, those will honor users settings that are configured, and they can also be configured by GPO. You can use PowerShell as well - but I believe it has to interact with WMI classes to do so.