A click on "Open" or on the file name opens the detail view of a document.
In a modal window you will see a preview on the left and on the right the file name, document date, the linked relation and the tags that have been assigned to the document.
If no relationship is linked yet, this can be done here too. All other data can also be changed, but once a relationship is linked, you cannot change it anymore.
To find a document easily, you can also add tags here. More about that under "Folders and Tags".
Caution: changes will only be saved after a click on "Save".