Quoting https://twitter.com/gvanrossum/status/533337876415533056

  @eugeneglybin Proprietary though it is, DOC[X] is easier to reflow to fit my screen. Also, I guess PDF fans don't use mobile screens enough.

Reflow isn't about PDF per se, it is about the underlying reader being used.

That said, it is certainly a much better alternative than DOC[X]. Different versions of Microsoft Word tend to produce different output for the same document.

With PDF, you are guaranteed the same output upon printing to any device; which isn't quite the case with DOC[X].