In order to complete this you must collect KING SLAYER FILES. However, not all docs are King Slayer Files. I am not sure if this is a glitch, or if only specific ones were intended to be.
King Slayer files can be docs found in the world (NOT ALL OF THEM THOUGH)
Legends collected in the world (NOT ALL OF THEM THOUGH)
OR
Videos watched after you complete missions (NOT ALL OF THEM THOUGH)
When I completed this acheivement, I was missing 2 files, and both of them were part of Ricky Sandoval's King Slayer files on the screen that shows all the defeated bosses.
4/5 on Inca Camina
(*DON"T WASTE YOUR TIME LOOKING FOR ANOTHER)9/10 on Media Luna
* From my research on the web, there are only 4 files for the base game in Inca. Even the guide released by Ubisoft shows only 4. The 5th apparently is part of the pre-order DLC - so don't waist your time looking for it like many others did.
In Media Luna, you will find (including in the guide) 10 files, but the middle top one (second down from the north), did not count - I checked after I collected each one knowing there was a problem in this province.
(NOTE: that there was a patch released yesterday, and perhaps this is fixed, but I will not know since I have finished).
I watch two different videos of people collecting this last file called "The Latin American Connection". Here is the video of the one that I found that got me the achievement.
This isn't mine. Dont' judge on the quality, but it worked for me and I wanted to share it. Perhaps they have fixed it.
ALSO NOTE: there are MORE files around the map which don't seem to count. And the official guide pictures posted in the solution above aren't completely accurate, or at least they weren't when I played in the first two weeks.
Hopefully this helps some.