My understanding is that adding a document to a SharePoint document library is not possible in PowerApps yet, but you can add an attachment to a list.
So my thought was first screen in the PowerApp is add the attachment to a SharePoint list, then after submit, then run a flow that creates that file in the library, then the navigates to a 2nd screen where the user could update the values on the SharePoint document library document.
Totally crazy to try to execute? I am a former InfoPath and SharePoint designer user, but very much for the business side, not the coding side.
Hmmm, I take images into a SharePoint library now, don't see why that same method can't work for other files. Anyway, I also think that SharePoint document library support has come out or is soon to come out. Your best bet would be go ask this in the actual Powerapps community. You would get much more Powerapp guru's eyes on it :).