code: public void setZoom(float min, float max) {
          if (min <= 0 && max < 0)
              return;
          PdfDictionary usage = getUsage();
          PdfDictionary dic = new PdfDictionary();
          if (min > 0)
              dic.put(PdfName.MIN_LOWER_CASE, new PdfNumber(min));
          if (max >= 0)
              dic.put(PdfName.MAX_LOWER_CASE, new PdfNumber(max));
          usage.put(PdfName.ZOOM, dic);
      }
docs: Specifies a range of magnifications at which the content
  in this optional content group is best viewed.
  @param min the minimum recommended magnification factors at which the group
  should be ON. A negative value will set the default to 0
  @param max the maximum recommended magnification factor at which the group
  should be ON. A negative value will set the largest possible magnification supported by the
  viewer application