This is the 10028th most frequent Russian word.
архивных
"Архивных" means "archival" in English.
Here, 'архивных' is used as a descriptor for 'записях', indicating that the records are related to archives—stored documentation.
Я нашёл интересные документы в архивных записях.
I found interesting documents in the archival records.
Here, 'архивных' modifies 'материалов', showing that the materials are from an archive, emphasizing the quality of being preserved.
В процессе изучения архивных материалов мы обнаружили важную информацию.
In the process of studying the archival materials, we discovered important information.
In this sentence, 'архивных' is an adjective describing 'документов', specifying that the documents are archival in nature.