This document is in the caverns near the entrance to the San Cordoba Challenge Tomb. If you missed it, fast travel to the Cenote Temple Ruins Base Camp.
From the campfire, head down the slope to the east, bearing right at the square column. Continue down the slope to the ledge where you originally climbed up.
Jump down onto the ledge below and move to the left edge, overlooking the chasm.
Now you're aiming for the path on the other (northeast) side of the chasm. Even with the screen brightened considerably, it's hard to see where to go. Check this map detail or just aim to the right of the tall column and Lara should land on the path.
To the right is the ledge where you first climbed up from below. Turn left...
...and squeeze through the gap between the rocks to emerge near the Challenge Tomb entrance.
Turn left (northwest) and you'll find the document sitting on the ground near a ruined cannon and some conquistador skeletons.
To get back to the Cenote Temple Ruins Base Camp, retrace the same route you took when playing through the story: Climb the wall to the west, shimmy along the narrow ledge, jump across the chasm to the climbing wall, rappel down, wall run and jump to the left, then work your way up and around to the camp. (Check these screenshots from the main walkthrough if you need a refresher.)
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