If you use a cloud based portal, they may have an option to notify the client via email when you save the tax return to that portal.

I use SmartVault and save all clients tax returns to their tax folder once they have been accepted. There is an option to "Send Income Tax Letter" email that advises them that the taxes have been filed and accepted.

I am able to modify the template letter to say what I want.

From there, they can log into their SmartVault folder and view the tax return.