The document APIs live on the workflow context. Therefore all files are stored relative to the workflow application they belong to.
Documents are valuable artifacts of workflow process. They must be included in backups and so forth as a consequence workflow administrators must known where these documents live.
.... so, I if the API would allow to store files just anywhere on a drive, this would be a miss-conception.
What you can do since 8.0 is to change the root directory of all files that belong to an application:
https://dev.axonivy.com/doc/8.0/engine-guide/configuration/files/app-yaml.html
![alt text][1]
[1]: https://answers.axonivy.com/upfiles/Selection_827.png