How does an instructor upload scanned submissions for an assignment and then grade them digitally in canvas?

Use case:

I have collected paper submissions of assignments from students. I have scanned the submissions into several .pdf files. Then I .zip them.

There appears to be a way to (re)upload submissions:

https://community.canvaslms.com/docs/DOC-12803-4152640871 

but in my canvas assignments there is no button to "upload submissions" 

What am I missing?

This seems like it would be a no-brainier piece of functionality for Canvas. The online grading tools are very slick but as always the hardest thing to do in Canvas is file-management (in this case, just getting a file into Canvas). Why have all the slick digital grading tools if you can't use them?

Thanks!

Will