If you are not already logged onto the portal you would get a logon prompt when accessing this URL and afterwards the resource will be opened. However if anonymous access is enabled you may not be able to access the resource and might encounter an error such as "Error 404 Resource not available".
In such a scenario it's important to consider the permissions assigned to specific resources, in the above example if the anonymous user doesn't have at least read permission to abc1.txt then they will not be able to view this resource. It doesn't matter that your specific user has read/write or even Full Control permissions on abc1.txt, when you click on such a link and you are not already logged on, you are accessing it anonymously and not authenticating with the portal.
Note it is never recommended to give anonymous users write access as a workaround in such a scenario as this could cause problems if multiple anonymous users were editing content at the same time.
This is a known limitation and is documented in +SAP Note 837898 - How to configure anonymous CM access +but I suppose this can be overlooked sometimes. You can still however use your UWL inbox to view your notifications and access resources for which notifications have been created. If you are just using URLs outside of notifications, e.g. providing a link to a KM resource from another file or location outside of KM then you could use a different servlet path to the resource, e.g.
