I figured it out for ProSeries and Link, and I don't know how ProConnect is different, but it might be. 

Once my client has accepted my invitation, I click on my client's name in Overview. Then I click on the Documents tab on the client details page.  I then click the blue Upload to Client button, and it gives me the option to attach a file - I attached my own checklist.  Then I clicked on Notify Client, and I can edit the stock message that is sent with my document.  

I tried this on myself as a client, just to make sure it works, and it seems to.  Let me know if it will work for you.