how to bundle several single pdf-documents into one big PDF-document

Do you have AEM forms OSGI?

if yes I can point you to some examples 

we are using AEM Forms JEE.

Ok.Are you familiar with DDX?

if not please refer to this document.

https://helpx.adobe.com/pdf/aem-forms/6-2/ddxRef.pdf

The ddx describes how you want to assemble pdf files 

then use invoke ddx component in workbench to create a process to assemble document

https://help.adobe.com/en_US/livecycle/11.0/WorkbenchHelp/WS92d06802c76abadb-1cc35bda128261a20dd-7fb5.2.html

https://help.adobe.com/en_US/livecycle/11.0/Services/WS92d06802c76abadb712193f912dbeb3c6db-7feb.2.html