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.
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.
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/