This article is from the PGP FAQ, by Jeff Licquia email@example.com with numerous contributions by others.
You might try adding "+pkcs_compat=0" to your command line as follows:
"pgp -seat +pkcs_compat=0 <filename>" By default, versions 2.3 and
later of PGP uses a different header format that is not compatible
with earlier versions of PGP. Inserting this option into the command
will force PGP to use the older header format. You can also set this
option in your config.txt file, but this is not recommended, as the
newer versions of PGP cannot understand the old signature format.