If the recipient group are already on your discussion list as double opt-ins or whatever the rules are, I’d think this might allow you to create a separate mailing list that you could then attach a pdf to and send out using regular mail.
If the discussion mail doesn’t have the functionality for attachments I’d certainly put the case to Support. If they remain adamant that it isn’t possible, you could create a simple password protected page on a subdomain that your readers could login to in order to download the pdf.
Or there’s always Google Mail
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