From Examiner.com
When Playing as Terra
Xehanort Report #2 – You will get Xehanort Report #2 automatically after beating the second boss in Radiant Garden.
Xehanort Report #5 – This report is awarded for completing Weaver Fever mode in Mirage Arena.
Xehanort Report #8 – This report can’t be missed. It is given to you after you defeat the boss during your return visit to Land of Departure.
Xehanort Report # 11 – You will acquire Xehanort Report # 11 automatically after beating the final boss in Terra’s story at Keyblade Graveyard.
When Playing as Ventus
Xehanort Letter – This letter is automatically obtained after defeating the orbs of light enemies in Land of Departure after you chose to play as Ventus.
Xehanort Report #1 – This report is inside of a treasure chest in the Launch Deck area of the Deep Space world. To get to it, use the anti-gravity device to make the crates float and then maneuver your way to the treasure chest located on the east wall of the area.
Xehanort Report #9 - You will receive Xehanort Report #9 automatically after a scene in Destiny Island.
Xehanort Report #12 – This report is located inside a treasure chest in the Seat of War area of Keyblade Graveyard. To the right of where you started is an area surrounded by big cliffs. Xehanort Report #12 is located on the northern wall.
Xehanort Report #10 - You will acquire this report automatically after beating the final boss in Ventus’ story at Keyblade Graveyard.
When Playing as Aqua
Xehanort Report #6 – You will receive Xehanort Report #6 automatically after defeating the boss in Enchanted Dominion.
Xehanort Report #3 – This report is inside a treasure chest in Merlin’s House on Radiant Garden.
Xehanort Report #4 – You will get Xehanort Report #4 automatically after a scene in Mysterious Tower.
Xehanort Report #7 - You will acquire Xehanort Report #7 automatically after beating the final boss in Aqua’s story at Keyblade Graveyard.