Should a user have a subscribed version of Adobe XD to receive a wireframe document from a user who has a subscribed version already?
Thanks for reaching out. There are certain limitations with a starter plan(free) which you can refer to here: https://www.adobe.com/products/xd/pricing/individual.html
To preview the prototype, a paid plan is not required. Feel free to reach out if you need any further assistance.
If you're sharing a link, the end user does not even need the Adobe XD app. It's merely a web link they open in a browser. If you want them to be able to open the XD file, edit it, etc then they would need the XD app (see @Rishabh_Tiwari 's link about the starter plan, which may be enough for their needs).