code: 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);
      }
docs: Sets the display duration for the page (for presentations)
  @param seconds   the number of seconds to display the page. A negative value removes the entry
  @param page the page where the duration will be applied. The first page is 1