Hi Steve,
As Kathy mentioned, you can select Full Page when exporting to PDF and it will put Adobe comments on the PDF. Some things we've noticed:
It takes a LONG time to generate the PDF
Comments aren't "mapped" to the page. So if the comment says "Comma goes here." You aren't going to see where here is.
If you've attached a file to a comment, it won't be accessible. It just says "[File attached]"
Also, you wouldn't need to give paid licenses to the external people. If you don't care if they hit the resolved check mark, you just send them a link to access the proof and they'd access it for free. (Side note: if you want them to make new comment or reply to other comments, I'd give them Reviewer permissions).
We do have one vendor who said for they would not do that, so that doesn't help in that case, but for those willing to go inside the proof, we definitely don't have to give them a paid license. (Again though, if you'd like them to "resolve" the comment, then they do need a proof license).