This article is from the PGP FAQ, by Jeff Licquia email@example.com with numerous contributions by others.
It is not secure at all. There are many ways to defeat it. Probably
the easiest way is to simply redirect your screen output to a file as
pgp [filename] > [diskfile]
The -m option was not intended as a fail-safe option to prevent plain
text files from being generated, but to serve simply as a warning to
the person decrypting the file that he probably shouldn't keep a copy
of the plain text on his system.