I worry—more than I should—about uncontrolled documents getting sent through corporate email. Documents should be like the immortals from Highlander:
Whenever possible, I put my documents on a shared drive and share the link. This is especially true when there is the expectation of collaboration. We don't need to spend time reconciling four different versions of a spreadsheet. Should I need to attach a document for someone without access to the shared resource, I'll send a .pdf: it's read-only.