参考资料：
https://stackoverflow.com/questions/33146021/generating-pdf-with-wkhtmltopdf-and-download-the-pdf#
https://github.com/topics/html-to-pdf
https://www.petrikainulainen.net/programming/spring-framework/spring-from-the-trenches-creating-pdf-documents-with-wkhtmltopdf/
http://www.laozhaimen.com/article/143508.html