Google update: Share Docs, Slides and Drawings with people who do not have a Google Account
When a user directly shares with individuals who do not have Google Accounts, those recipients will be able to view the file without signing in. Because no sign in is required, anyone may view the file with this sharing link until the person who the file was explicitly shared with creates a Google Account and expends the invitation. Once the person creates a Google Account two things happen: (1) the sharing link will no longer work for new users to access the file and the sharing dialog will indicate that the invitation has been used; (2) any user who accessed the file using the sharing link while it was open and signed in using their Google Account will be added to the sharing access list for that file and will continue to have access. Users with permissions to change sharing settings can revoke this access if desired.
Google Apps admins can prevent this behavior by disabling sharing outside the domain to people who are not using a Google Account via a setting in the Admin console.