Google Drive is Google's file sync app that lets you store all of your files online alongside your Google Docs documents, and keep them synced with all of your devices.
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.