Have you read the F.A.Q.? It is linked to on the home page as well as the landing page linked to above. These booklets are password protected because they are for fixed time contests, and the idea is you do not see the puzzles till your timer starts, so that no one gains an unfair advantage. So any particular contest, once it starts or once you click the start button, the password will appear and a timer will begin to count down, at which point you are expected to print the PDF and start solving.
Again, this is all explained clearly with images in the F.A.Q. which I highly recommend going through.