Several of the books that were previously only available as PDF can now be downloaded as ePub, for anyone wanting to transfer/convert them for another reader.
They are: Collision Course, Colters' Woman, Hara's Legacy, Long Hard Ride, Standoff, The Ghost Exterminator, and Trey.
You'll have to go back to the promo page and re-add the those titles to your library one-by-one, which should give you duplicates in your My Library listing from which you can choose to download either ePub or PDF in the future.
If doing it directly from the promo page doesn't work, then try clicking each book's product page link from the promo and trying the Add to Library button on the individual pages, which should.
Also, this promotion officially lasts until the 12th, according to the rotating banner that was on the Borders front page.