You have 5GB of online storage for your personal documents which you can fill with however many Kindle ebooks or PDFs you wish. Any notes that you add to the uploaded docs will also be saved and synced across the K3 and K4 (and Kindle Touch when it is released). I don't know about you, but that's a feature that I've wanted ever since the original Kindle.
BTW, this is very good news for anyone who likes the keyboard on the K3. It's a sign that Amazon plan to keep the K3 around indefinitely as one of their ebook readers. After all, they wouldn't have gone through the expense of updating the firmware if they didn't plan to continue to sell it.
Now, Amazon did say at the press conference that they were keeping the K3, but I wasn't really sure what they meant. The previous Kindle models were still around for some months after they were replaced, and the K2 was in fact still available as a refurb almost a year after the K3 launched.
So it's my guess that the K3 will stay on the shelf until at least next year. Amazon will likely launch new and updated Kindles at some point, and that's probably going to be when the K3 is EOLed.