In ACT! 2011, you could add a file to the Documents tab and it would also include a history of “Library Document Attached” (Coincidentally, if you deleted one, it would delete the other). It changed in 2012 (and therefore also 2013). Adding a file to the documents tab no longer also creates a history entry. This was a very useful feature as it kept ALL history in one place. A user did not have to look in both the History and the Documents tab for items related to the current contact.
It was an unfortunate feature to remove from the product. It was most helpful to people who use QuoteWerks so that when a quote was attached to the Documents tab, it was also recorded in History. ;-(