I have already posed this question in the Flow forum and was informed this funtionality is not currently supported i.e. it's not possible to retrieve the URL of an SP document hosted in an SP Doc library.
This would be really usefull, given that it's not possible at the momment to have an attachment in PowerApps, and to upload that attachment to a Sharepoint Doc library.
So as a work-around, the User could upload a Document to an SP Doc library, a Flow in the Doc library could move the Doc to the Final Doc library (need to move doc to another library because of permissions), if it was then possible to retrieve the new URL of the document, we could save the URL in an SP list, so that the User would be able to click on the link to open the Doc in PowerApps.
There are many other scenarios where having the URL of the Doc would be really helpfull. So please add this to the Dynamic content that is available as part of Flow.