You don't need this book to play Hell on Earth Reloaded. This is a book you'd hand out to your players if you want to keep some semblance of mystery to the game, etc. It gives them just enough background information to get a sense of the setting, etc. The only difference is the PDF is setup so that you can turn images off and it's under a different license so that you can print as much as you want for your players, etc.

The regular Hell on Earth book is the complete book which has all this information (including spoilers of what's really going on).