code: public static String getFilePath(int docId, String extention) {
          int[] split = getSplit(docId);
          return split[0] + "/" + split[1] + "/" + split[2] + "." + extention;
      }
docs: @param docId
  usually PubMed id
  @param extention
  e.g. "gz", or "pdf" (WITHOUT dot!)
  @return a path, e.g.:
  <ul>
  <li>2345678, "gz" -> "2/345/678.gz"</li>
  <li>78, "pdf" -> "0/0/78.pdf"</li>
  </ul>