In reply to Kerning, Software, and other thoughts:
... – millions of legacy documents would reflow
That's not a valid reason. Word documents may already reflow by the simple act of opening them on another computer.
For some reason, Word insists on re-flowing text for the currently selected printer, taking its ppi and whatnot into account. While that's very printer-friendly (the hardware kind -- i.e., possibly intended to get the best possible output per desktop printer), it results in printers (the people) accepting a document from a client, opening it to print to PDF, and then seeing headings at the bottom of the page, page numbers in contents and indexes off, a totally different number of pages, etc.
Back in the day when we used to accept "print-ready" Word documents I always had a blast, moving from one printer setting, one Word version, and one computer to another, attempting to find the magic combination that suddenly made the document at least appear correct.
We now circumvent these issues by telling clients to generate a PDF themselves. Not a thankful job either (as the typical Word user doesn't have a clue of what a Virtual PDF Printer is, where to find it, and why their document may reflow when they install and select it) but at least we moved the problem off our desk towards the client.