The easiest way to do this would be to:
You can publish a service that gives a System.FileDocument type of response.
In your service;
- Retrieve file document that you want to send
- If found, return that object
- Otherwise, adjust your httpResponse and send empty object
You are asking for a solution to share files. To be honest, sharing files via API is not a common way.
Better and more easy is to use a shared file storage system (e.g. Amazon S3) use this for both applications. You can use the FTP (file transfer protocol) to upload/download files. All files changes are directly available for both apps.