Liferay 文档 PDF 水印相关讨论
http://www.liferay.com/documentation/liferay-portal/6.1/user-guide/-/ai/documents-and-media
http://forum.primefaces.org/viewtopic.php?f=3&t=14553
http://www.liferay.com/community/forums/-/message_boards/message/9975028
http://www.liferay.com/community/forums/-/message_boards/message/12328654