* @param page the page where the duration will be applied. The first page is 1
     */
    void setDuration(int seconds, int page) {
        PdfDictionary pg = reader.getPageN(page);
        if (seconds < 0)
            pg.remove(PdfName.DUR);
        else
            pg.put(PdfName.DUR, new PdfNumber(seconds));
        markUsed(pg);
    }