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.
Merge linked report PDFs by page order and reply with one file link.
Your teammates get a combined report PDF in their requested page order, with a link back in the email thread.
Merge each request's PDF links into a single filed document.
Your reports or invoices become a single PDF in your shared folder, with a link on the request you submitted.