From the back of the adventure:
Near the sleepy town of Winterhaven, a ruined keep overlooks a dormant gateway to the otherworldly realm knoown as the Shadowfell. Though this dark scar on the world has lain dormant for many years, an evil cleric of Orcus, Demon Prince of Undead, seeks to reopen the gate. The only thin preventing the forces of darkness from sweeping across the land is a determined band of heroes...
Contents: 80 page adventure book, 16 page 4th Edition quickstart rulebook, 5 ready to play 1st level characters and three double-sided battle maps.
An explanation of the three different (English language) versions
The original print release (listed as "First printing" here) in 2008 was the first adventure released for Dungeons & Dragons 4th Edition and was designed as an introduction to the game. It came in the folder-pack typical of early 4e adventures, and contained a 16-page set of quickstart rules and pregenerated characters, allowing someone to use it without the full 4e rules.
In 2009, Wizard of the Coast released a revised version of the adventure as a free PDF on their web site. This was just the adventure booklet, and was updated to include more developed skill challenges and several revised and rebalanced encounters. At the time of writing, this version is still available as a free download.
In 2013, when the DNDClassics web site was launched to start selling PDF versions of Dungeons & Dragons products, one of the products available for sale was "H1: Keep on the Shadowfell & Quick-Start Rules". Although it is listed as having a page count of 145 pages here, it is identical to the original 2008 print release. The additional pages are due to the rather clumsy scanning of the three double-sided battlemaps that came in the original folder, rather than any new content.