Here is why that specific piece of paper was so significant:
: Spencer realizes the blueprint is dated months before the construction of that wing actually began.
Another notable paper trail in this episode involves the Hanna finds in Wilden’s car, which places him in a different location than he claimed during the time of the "remains" discovery.
: It also deepens the mystery surrounding Toby’s mother, Marion Cavanaugh, suggesting that the "accidents" at Radley were planned or covered up long in advance.