This article is from the PGP FAQ, by Jeff Licquia firstname.lastname@example.org with numerous contributions by others.
The fact that the entire source code for the free versions of PGP is
available makes it just about impossible for there to be some hidden
trap door. The source code has been examined by countless individuals
and no such trap door has been found. To make sure that your
executable file actually represents the given source code, all you
need to do is to re-compile the entire program.