One should complete this side objective after visiting the Partisan HQ.
The graveyard is located by the church (M2.8). The easiest way to get there is by reaching the second floor of the building by the port, and jump between the balconies (M2.10) - there will be no enemies around.
The document that you are looking for is located in the mausoleum. You can open it with a key (below (M2.7)), located inside the church, on the table, near the main entrance from the courtyard.
An alternative way would be to blow up the wall of the mausoleum with any explosives (even a grenade).