This is the 3086th most frequent Russian word.
исторических
"Historical" or "of historical" (adjective form).
Here, 'исторических' (historical) is used as an adjective to describe the artifacts related to history.
Мы посетили музей, полный исторических артефактов.
We visited a museum full of historical artifacts.
In this sentence, 'исторических' describes 'событиях' (events) that pertain to history.
Книги о исторических событиях популярны среди читателей.
Books about historical events are popular among readers.
The word 'исторических' is here modifying 'документов' (documents), indicating these are documents of historical importance.