code: protected String stringValue(COSBase value)
      {
          if (value instanceof COSString)
              return ((COSString) value).getString();
          else if (value instanceof COSNumber)
              return String.valueOf(((COSNumber) value).floatValue());
          else
              return "";
      }
docs: Obtains a string from a PDF value
  @param value the PDF value of the String, Integer or Float type
  @return the corresponging string value