How to get latest revision of a document using Rest API ?

Could you please post the complete request here?

Including any headers for Post, that contain any additional parameters.

This might help in seeing if there's anything off in the request itself.

Assuming you have access to a windchill server and that windchill is accessed as: 

http://windchill.yourdomain.com:80/Windchill

you can access the below REST call with this URL 

http://windchill.yourdomain.com:80/Windchill/servlet/odata/DocMgmt/Documents?$expand=Revisions

As the above REST call is a GET call you can try the above URL in a browser and you will get the response in xml format.

You want to try non GET operations like POST, DELETE, PATCH, etc, with more control over your REST request, use a REST client like Postman. With tools like Postman, you can get the REST response in JSON also by adding header 

Content-Type: application/json