Quick answer: the Newsletter plugin does not support adding attachments included in your newsletters. We suggest adding a button (CTA – Call to Action) with a link to the file you want to provide to your subscribers storing it in your media library.
Even digital things have weight. If a newsletter can be compared with 1 sheet of paper, an attachment (for example a PDF) could weigh as 100 sheets.
Sending such an amount of data takes up time, resources, and bandwidth sometimes compromising a good delivery experience.
We suggest adding a link to your newsletter the subscriber can use to get the file “on demand”: that saves up even space in the subscriber’s mailbox.