code: public void setKeywords(final List<String> keywords) {
          StringBuilder builder = new StringBuilder();
          for (String keyword: keywords) {
              if (builder.length() > 0) {
                  builder.append(',');
              }
              builder.append(keyword.trim());
          }
          this.keywords = Optional.of(builder.toString());
      }
docs: The keywords to include in the PDF metadata.
  
  @param keywords the keywords of the PDF.