design – How to access a file stored locally on the server?

I am working on a web application that allows users to review PDF documents. These documents are sent from another public website. A typical workflow involves:

  1. A document is uploaded to the public website and stored in the document management system (DMS).
  2. The internal web application takes the document from the shared DMS and shows it to the user.
  3. The user of the web application downloads this pdf to his local file system.
  4. Add some marks / annotations / comments in the pdf.
  5. The revised document is sent to the request, which processes the document and presents the corrections to the user who sent the document on the website.

We were using a cloud service to manipulate PDFs so far, so there were no problems accessing the revised PDF, since the API handled it. But some of the clients have problems to store PDF files in the cloud, so I need to implement a local solution.

My question is, how can my web application know when the user has finished reviewing the document (which will probably be a save event), so that he can retrieve the document and process it? As far as I know, the server does not know anything and can not access the local file system.

I can probably force the user to load the revised document into the application, and that would make my life easier. But, from a usability point of view, and for a consistent user experience with the solution in the cloud; what allowed the communication in the event "save", I would like to have something similar, that is, when the user clicks save after reviewing, my application wants to capture the document for further processing.

I am not sure if there is any way to prevent the server from trying to access the local file system.

Note: it is a company web application on the premises.

TL; DR: my web application server would like to know when a document is stored locally and take that document.