Per yesterday's discussion of Mongoose Traveller's Book 0: Introduction to Traveller, I am sorry to say that it looks like both the print and the pdf will set you back at least a fiver (unless you were lucky enough to pick it up at 2008 Free RPG Day). That's a shame--I like a good, free quickstart product as much as anyone. Perhaps in the future, they'll offer the pdf for free. I'd be more apt to pick up Mongoose's pdfs in general if the prices came down. (Think Paizo. Think about the fact that I'm a lot more likely to buy the print copy of a book I enjoy in pdf form).
All is not lost, however. Mongoose does have an SRD and Developer Kit you can download (warning: direct zip link) from their downloads page that might give you a bit of an idea of what it's all about. Not as good as a product purely designed for an introduction, but better than a stick in the eye.
Here are a few introductory Traveller products (of differing quality) on DriveThruRPG that are free (none of them for Mongoose Traveller, mind):
-Understanding Traveller: The New Era
-Traveller20 Lite (I still think that's Randy Quaid on the cover)
-DTRPG Guide to MegaTraveller
My advice, no matter what version you choose, is always the same: don't let someone else define your Imperium for you. Traveller is the ultimate space sandbox; take advantage of that. The game you want to run is somewhere in Traveller; you just have to find it.
(Hello to Jon Brazer Enterprises, a company making Traveller products who I just met yesterday!)