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To download and upload a file using a RESTful API in Spring Boot, you can follow these steps:

1. Define the REST endpoint for file download by creating a controller method that accepts the file name as a path parameter:

   @GetMapping("/download/{fileName}")
   public ResponseEntity<Resource> downloadFile(@PathVariable String fileName) {
      // logic to retrieve the file from location and create Resource object
      // return ResponseEntity object with Resource object and HTTP status code
   }
   

2. Implement the logic in the downloadFile() method to retrieve the file from the source location and create a Resource object to return in the ResponseEntity. You can use the FileSystemResource class to create the Resource object.

3. Define the REST endpoint for file upload by creating a controller method that accepts the file as a RequestBody:

   @PostMapping("/upload")
   public ResponseEntity<String> uploadFile(@RequestBody MultipartFile file) {
      // logic to save the file to target location
      // return ResponseEntity object with success message and HTTP status code
   }
   

4. Implement the logic in the uploadFile() method to save the file to the target location. You can use the transferTo() method of the MultipartFile object to transfer the file to the target location.

5. Test the RESTful API endpoints using a REST client like Postman or cURL.

Note: You may need to configure the target file location and the file size limit in the Spring Boot application properties or in the controller methods. Also, make sure to handle exceptions and provide appropriate error messages in case of invalid file uploads or downloads.