You can download the CommunityCommons module from the appstore which includes an java action 'MergeMultiplePdfs'. With this action you should be able to accomplish what you want.
Another idea: use a REST service to convert files to PDF, then use community commons Merge PDF java action to combine them.
There are many online PDF conversion services, one that I found is here. These services charge a fee for the conversion.
Sometimes we want to merge multiple PDF files into single one. This issue can be solved by using a small utility Tool with Merge PDF Tool is a small open source tool, that can be used to merge several PDF files into Single PDF file. visit at:- https://www.softwarewiki.org/merge-pdf/