The answer is usually "create a field in the same object." This is sort of implied by the "rules" of database normalization (the First Normal Form, Second Normal Form, etc). You can do a search to learn more, but basically speaking, all data that isn't duplicated belongs in the same table, and data that would be duplicated should be in ...
Its may be late but I think will be helpful You can query the documents based on lastModifiedDate column like below.
select Id, Title, Description, ContentSize, isDeleted, OwnerId, LastModifiedDate, CreatedDate, FileType, PublishStatus from ContentDocument where OwnerId = 'XXXXXXXXXXXXXXXXXX' and LastModifiedDate > 'CURRENT_DATE' order by LastModifiedDate
I ended up with Approach 1 i.e created a 'new Object' with a master detail relationship to Account. On the Parent Account Detail page, created a related list to display records of 'New object'. This approach offers scalability and robust reporting capabilities.