Symbolism, Imagery, Allegory
The police report, authored by Gunnar Björck under the pseudonym Sven Jansson, is a symbol of everything that's wrong in Salander's life, and she's never even seen a copy of it until she finds the one Bjurman got his hands on. The report details Salander's attack on Zala when she was twelve. This resulted in her being placed in the psychiatric hospital for children and her life becoming a total horror show. It's been buried by the Secret Police, who want to keep Zala from becoming known to the public, so it also symbolizes the secrecy and corruption which also contribute greatly to Salander's wretched situation.
From the police report, Bjurman finds out that Zala and Salander are connected, and he contacts Zala, sparking the novel's major plot lines. It's also the key piece of evidence Salander uses to figure out that Zala's crimes are being actively covered up by the Secret Police, and that her confinement to St. Stefan Hospital's was no random accident. So, it also becomes a symbol of the idea that secrets always come to light sooner or later, especially if they are written down on paper.