Option 1 using an InputStreamResource

Resource implementation for a given InputStream.

Should only be used if no other specific Resource implementation is > applicable. In particular, prefer ByteArrayResource or any of the file-based Resource implementations where possible.

@RequestMapping(path = "/download", method = RequestMethod.GET)
public ResponseEntity<Resource> download(String param) throws IOException {

    // ...

    InputStreamResource resource = new InputStreamResource(new FileInputStream(file));

    return ResponseEntity.ok()
            .headers(headers)
            .contentLength(file.length())
            .contentType(MediaType.APPLICATION_OCTET_STREAM)
            .body(resource);
}

Option2 as the documentation of the InputStreamResource suggests - using a ByteArrayResource:

@RequestMapping(path = "/download", method = RequestMethod.GET)
public ResponseEntity<Resource> download(String param) throws IOException {

    // ...

    Path path = Paths.get(file.getAbsolutePath());
    ByteArrayResource resource = new ByteArrayResource(Files.readAllBytes(path));

    return ResponseEntity.ok()
            .headers(headers)
            .contentLength(file.length())
            .contentType(MediaType.APPLICATION_OCTET_STREAM)
            .body(resource);
}

Try this:

@RequestMapping(value="/viewAttach", method = RequestMethod.GET)
public ModelAndView viewAttach(@RequestParam(value="article_id", required = true) String article_ref, HttpSession session, HttpServletResponse response) 
{

    /* *** Check Session *** */
    try {

        // Direclty from pier.content.GetContent written by ang94402

        URL url = new URL(binaryURL);           
        response.setHeader("Content-disposition", "attachment;filename=" + binary.getName());

        //Set the mime type for the response
        response.setContentType("application/pdf");

        // URLConnection connection = url.openConnection();
        InputStream is = url.openStream();

        BufferedOutputStream outs = new BufferedOutputStream(response.getOutputStream());
        int len;
        byte[] buf = new byte[1024];
        while ( (len = is.read(buf)) > 0 ) {
            outs.write(buf, 0, len);
        }
        outs.close();

    } catch (MalformedURLException e) {
        logger.error("Error ModelAndView.viewMain - MalformedURLException : " + e.toString() + " -- " + e.getStackTrace()[0].toString());
        return null;
    } catch (IOException e) {
        logger.error("Error ModelAndView.viewMain - IOException : " + e.toString() + " -- " + e.getStackTrace()[0].toString());
        return null;
    }


    return null;

}

@RequestMapping(value = "/files/{file_name}", method = RequestMethod.GET)
public void getFile(
    @PathVariable("file_name") String fileName, 
    HttpServletResponse response) {
    try {
      // get your file as InputStream
      InputStream is = ...;
      // copy it to response's OutputStream
      org.apache.commons.io.IOUtils.copy(is, response.getOutputStream());
      response.flushBuffer();
    } catch (IOException ex) {
      log.info("Error writing file to output stream. Filename was '{}'", fileName, ex);
      throw new RuntimeException("IOError writing file to output stream");
    }

}

Generally speaking, when you have response.getOutputStream(), you can write anything there. You can pass this output stream as a place to put generated PDF to your generator. Also, if you know what file type you are sending, you can set

response.setContentType("application/pdf");

In your controller method you can add this code to get file download

File file = new File("fileName");
FileInputStream in = new FileInputStream(file);
byte[] content = new byte[(int) file.length()];
in.read(content);
ServletContext sc = request.getSession().getServletContext();
String mimetype = sc.getMimeType(file.getName());
response.reset();
response.setContentType(mimetype);
response.setContentLength(content.length);
response.setHeader("Content-Disposition", "attachment; filename=\"" + file.getName() + "\"");
org.springframework.util.FileCopyUtils.copy(content, response.getOutputStream());

You need to use a HTTP client to download the file you want to proxy. Here is some example how you can do this. This example uses the Apache HTTP client 4.5.

@RequestMapping(path = "/downloadFile", method = RequestMethod.GET) 
public ResponseEntity download(String param) throws IOException { 
    String testFileName = "https://www.google.co.jp/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png" 
    HttpClient client = HttpClientBuilder.create().build();
    HttpGet request = new HttpGet(testFileName);

    request.setHeader("Cache-Control", "no-cache, no-store, must-revalidate"); 
    request.setHeader("Pragma", "no-cache"); 
    request.setHeader("Expires", "0");

    HttpResponse response = client.execute(request);
    if (response.getStatusLine().getStatusCode()==200) {
    ByteArrayResource resource = new ByteArrayResource(Files.readAllBytes(path));

    return ResponseEntity.ok()
        .headers(headers)
        .contentLength(file.length())
        .contentType(MediaType.parseMediaType("application/octet-stream"))
        .body(response.getEntity().getContent());
    } else {
        return ResponseEntity.notFound();
    }
}