get Google Doc from id just retrieved

I am developing a Java - JSF web application in which I am showing a pop-up in order to let the user choose a document from Google Drive to download it.

To do that, I have some js code:

<script src="filepicker.js"></script>
<script>
    function initPicker() {
        var picker = new FilePicker({
            apiKey: 'MY_API_KEY',
            clientId:  my_client_id,
            buttonEl: document.getElementById('pick'),
            onSelect : function(file) {
                        if (file.id) {
                            sendLinkToBean(file.id);
                        } else {
                            alert('Unable to download file.');
                        }
                    }
                });
    }
</script>

<a4j:jsFunction name="sendLinkToBean" action="#{gPPersonFormBean.downloadFile()}">
    <a4j:param name="param1" assignTo="#{gPPersonFormBean.fileId}"/> 
</a4j:jsFunction>

The file.id arrives the Bean and I try to get it as shown in G.Drive's API:

public void downloadFile(){
    try {
        Drive driveService = getDriveService();
        OutputStream outputStream = new ByteArrayOutputStream();
        driveService.files().get(fileId) .executeMediaAndDownloadTo(outputStream);
    } catch (IOException e) {
        String mess = "downloadFile(): " 
                + (e.getMessage()!=null?". "+e.getMessage():"")
                + (e.getCause()!=null?". "+e.getCause():"");
        logger.error(mess);
    }
}

But I get FileNotFoundException:

com.google.api.client.http.HttpResponseException: 404 Not Found
{
 "error": {"theID"
  "errors": [
   {
    "domain": "global",
    "reason": "notFound",
    "message": "File not found: 1tvnGWDCse4TFQwIvqEDTlvv2cebfs0C2JBtjTP5A42I.",
    "locationType": "parameter",
    "location": "fileId"
   }
  ],
  "code": 404,
  "message": "File not found: theID."
 }
}

Does anyone know why this might be happening? Thanks in advance.

EDIT: I have just compared the id received with the one given by Google with the file's link and they are exactly the same.

EDIT 2: If I do driveService.files().list(); it returns an empty list.

As Tanaike said I had to use export instead of get but also executeAndDownloadTo instead of executeMediaAndDownloadTo

So it worked being:

String fileId = "## file ID ##";
OutputStream outputStream = new ByteArrayOutputStream();
driveService.files().export(fileId, "application/pdf")
    .executeAndDownloadTo(outputStream);