Depending on Document Status
When a Document has just been Created only Document Authors can upload a file to a Document, this means that only Author(s) will see the Upload button when a Document is in Created Status.
Once a file has been uploaded the Edit button will appear. If there is no file to edit the button is not visible.
Once a Document reaches Under Signing by Approver Status the Edit button is no longer visible.
Depending on Roles in the Document
Managers, Regular Users and Read-Only Users will only see the Edit button if they have a Role in the Document.
Administrators see the Edit button regardless of having a role in the Document or not.