Without a secure file box, you should turn to your cloud-storage service of choice. Upload the file and use the built-in file-sharing features to send your recipient a link. This is safer than sending the file as an email attachment, since the recipient's email service may not have strong security. By sharing the file through something like Dropbox, you at least know it's traveling over HTTPS, so other people on the network can't see it, and you'll be able to remove the file from your cloud storage after the recipient downloads it. This method isn't perfect (since, again, Dropbox can see your files), but it's almost certainly better than using an email attachment.