The out of the box hehaviour of Alfresco is protect the access with a user and password. Anyway, after user is logged you'll be redirected to the details of the document.
On the other hand, if you want that guest user can access to your document, you must assign permissions. After that, the url will be something like that:
Anyway, I think isn't a good practice that any user have access to our documents.
Maybe you could develop a "pre-system" to manage this issue. Por example a script that obtain a valid ticket and after that redirect to the details of the document.
If the previous url is correct, we will obtain a valid ticket that we can use to access: