Google Contacts, the address book built into Gmail, lets you keep track of all your contacts, see when you last interacted with them, and more.
Combine report pages in order and link the finished PDF to its request.
Your report or invoice packet is ready on the same spreadsheet row as the source PDFs, with its pages in the order you chose.
Assemble report pages into one PDF and send its link to the requester.
Your teammates get a combined report PDF in their requested page order, with a link back in the email thread.