You best best would be that while splitting the PDF, save them into folders named after the customer's email. This way, all PDF for a specific customer will be together.
Once all splitting is done, use the
Folder Listing plugin to get a list of all the PDFs that were created and their folder name.
Split te resulting file from the
Folder Listing plugin (it is an XML file) per folder (previously named after the customer's email).
Now that split will act as a loop. Inside that loop, use the
ZIP Job File or folder content plugin.
The resulting of that plugin is a ZIP file containing all PDFs for a specific customer which you can then attached to your email.