Ghost now offers a tutorial on Lead Magnets/file upload (& partially-hidden pages, a la Substack): How to use lead magnets to grow an audience
You can modify the default email signup on the Page editor…
And if you’re looking to send it in a welcome email on signup (requires some Zapier/emailer integration): Custom email series: welcome / retention / renewal
Or just send the user to a Welcome Page (this is built in!): Ghost email subscription welcome page/autoresponder - #8 by gjdickens
Hope this saves y’all some time like it did me!