You can try to use some package like this one.
https://packagist.org/packages/grofgraf/laravel-pdf-merger
But if your PDF files is protected against modification, I don't know any free API to merge them. For those files you need to break the protection.
This API propose a code to break the protection, but only if you are authorized to modify a PDF file (you have to be the owner of the PDF file).