Is there a way to present a given document to every user the next time they visit? They would only see it once, and any subsequent visits, they see SharePoint as normal. This would also be something that could be easily re-used on additional documents.
You could add a custom control on the master page to display the link to this particular document, e.g. on the top of the screen.
This kind of "Special Announcement" control would have to implement the rest of the logic. When would it be visible? Should it be hidden if the user has seen the announcement, or if has clicked through? You could keep track of that using a central list, and use a second list to store the current announcement.
Bare in mind that this puts additional load on EVERY page. Also, if the users are using links directly to e.g. document libraries, you might want to change the System Master Page too.
Does it really need to be a document.Can the content be in a list? You can use data from a list and use jquery to load it and of course a session to make sure they don't get it again or anytime you load it store there username and and they use that to check if they have seen it before. Cheers Patrick