This is no different than Google Docs and many other similar services. If you share a Google Doc with someone it will show up in their list of Docs. But it is j

This is no different than Google Docs and many other similar services. If you share a Google Doc with someone it will show up in their list of Docs. But it is just a reference to "their" doc. If they delete the doc there's no doc to reference and your "apparent" copy of the doc (which was really just a reference) disappears. If you really wanted your own personal copy you need to make a duplicate. That duplicate will be yours not a reference to theirs. It was strange the first time I ran into it. I deleted an account. I had shared the documents with another account of mine. Then I noticed when I deleted the first account I lost access to those docs. Now I know I hadn't copied the docs I'd only had references to them. Next time I'll know to make copies if I actually want copies and not just access to another account's shared docs.