Hi there,
If the other user opens the PDF the same way you did, and activates Hypothesis (in the event other forum members read this,
the process is here), your annotations will be visible to them. They'll only see the file as unavailable because if they try to view annotations in context from the
Activity Page. So you
could email your users a copy of the PDF as well as the local PDF instructions.
However, there are other alternatives.
Https://docdrop.org will store documents and even embed Hypothesis on the same page as the document for you automatically. The URLs these documents reside at will not be private; anyone can view them (although if your annotations are in a private group then the annotations themselves remain private).
I've also seen people host documents on WordPress sites and similar. Any place where the document can be displayed directly, without appear inside another site's frame (the "wrapping" around the PDF you'll see on Google Drive, OneDrive, Dropbox, etc).
Best,
Michael