将一组PDF文档合并（服务器端）到JAVA中的一个大PDF文档的最简单方法是什么

I have 3 PDF documents that are generated on the fly by a legacy library that we use, and written to disk. What's the easiest way for my JAVA server code to grab these 3 documents and turn them into one long PDF document where it's just all the pages from document #1, followed by all the pages from document #2, etc.I have 3 PDF documents that are generated on th