Trying to use PGP in this instance is a lost cause. You risk missing a deadline, you risk that your file doesn't reach the right recipient, you risk that you will be frowned upon, etc. If they encouraged encrypted submissions, they would probably make their public key accessible. Since they don't make the key available (I assume you've done your research), still to what's conventional in the business. Either email unencrypted or send via snail mail (using a courier service).