I downloaded my files, but they're in epub. It was so long ago that I'd forgotten they only sold them in one format. What in the world?
(I'm pretty sure they're non-DRM, so I should be able to convert them.)
They're also on my Amazon Content and Devices. If I go to the page for one of the books, it says "You already borrowed this item. Read anytime on your Kindle apps and devices."
On the Amazon page, there is also a "Buy at Pottermore" link. Whoops.
There is also a Buy link in case I want to buy the ebooks again.
Those links go to the newer editions with different covers.
I
think when Pottermore was selling the exclusive ebooks, Pottermore and Amazon used to have an agreement so that if you bought the books at Pottermore, you could download them via Amazon for your Kindle. Once JKR allowed other stores to sell the eBook, that changed, and the ebooks were available for purchase through Amazon, B&N, etc.