I was under the assumption there is (going to be) a general user tracking module? If not, would you consider to write such a module, which stores the user, the object_id, the number of times the object has been visited by the user and the last_time the object has been seen by that particular user or would this be out of scope?
Then you could maybe enable a service contract with file storage that add's an entry (or increases it) for that particular object.
In the long run you would not only have the valuable information, which user has viewed what document, but also what documents are the most popular. This would be a valuable addition to an universal grading system in that sense, that you could see, what object has been viewed by which user, but not yet rated by him. It would be especially interesting to get his/her rating/comments, if the file was downloaded a couple of times. But I'm sure I'm way out of scope here :).
So, my request / suggestion would be to keep the table as general as possible and use a counter with this table. If you can go one step further, make a seperate service package out of it. And make sure you can turn this off site wide and on a per user basis (if your client is keen on getting this data, but not all his users agree to it, and have the legal right to decline the collection of this data).
Last but not least, thanks for letting us know and getting the discussion out in the open!