@Zephyr flutist: If someone doesn't open all chests or simply doesn't find one it is possible to miss more than one report.
@Narnianqueen:
Ok, here we go (I refrain from using spoiler-tags this time cuz the locations of the reports don't spoil anything):
- Xehanort's Letter => obtained by Ventus when clearing Land of Departure.
- Xehanort Report 1 => obtainable by Ventus, look for a chest in the launch deck in Deep Space.
- Xehanort Report 2 => obtained by Terra when defeating the boss at Radiant Garden.
- Xehanort Report 3 => obtainable by Aqua, look for a chest in Merlin's house at Radiant Garden.
- Xehanort Report 4 => obtained by Aqua after having a conversation with Yen Sid at Mysterious Tower.
- Xehanort Report 5 => obtainable by Terra, beat the "Sinister Sentinel"-Event at the Mirage Arena.
- Xehanort Report 6 => obtained by Aqua when defeating the boss at Enchanted Dominion.
- Xehanort Report 7 => obtained by Aqua after defeating the final boss of her scenario.
- Xehanort Report 8 => obtained by Terra when defeating the boss at Land of Departure.
- Xehanort Report 9 => obtained by Ventus after visiting Destiny Islands.
- Xehanort Report 10 => obtained by Ventus after defeating the final boss of his scenario.
- Xehanort Report 11 => obtained by Terra after defeating the final boss of his scenario.
- Xehanort Report 12 => obtainable by Ventus, look for a chest in the Keyblade Graveyard at the seat of war.
EDIT: Corrected mistake for Xehanort Report 5, on Weaver Fever Terra gains HP +5, not a Xehanort Report.