We get .docx documents from a backend server. The users downloads them to their local machine, edits them jusing MS Word/Excel/... and uploads them back.
Is it possible to make angular open the installed Word and register when the user saves the document so that it can upload them back into the backend server?
No, it is not possible. To be precise, only Internet Explorer understands the COM technology on Windows. Other browsers will not let you automate the locally installed desktop Office applications like Word.
As a possible workaround you may consider using protocol handlers for Office applications.