Wordpress - Gravity forms - Develop functionality to merge a set of PDFs uploaded via gravity form

We are a web development company building custom solutions for our clients using Wordpress. THE CONTEXT We are building a site where users can create a legal documents filling out a form with specific fields (using Gravity Forms plugin). When the user submits the form, an entry with all the data is created under the admin panel (wordpress). The GravityPDF plugin creates a PDF file using a template file we have created already, and handles the presentation and look and feel. THE PROBLEM TO BE SOLVED In the form, there are some cases where the user must upload a set of PDFs files. The desired functionality should take all these PDFs files uploaded and merge them with the actual document created by the plugin (GravityPDF). The ONE PDF that GravityPDF creates with the merged files. REQUIREMENTS The final code should be implemented either on a custom Wordpress plugin or in the [login to view URL] file. We will set a testing server where all the changes will be made.