personal document box; small stationery box
A small, often portable box used to store papers, letters, or writing supplies. Historically common before modern filing systems.
祖母の手文庫には古い手紙がたくさん入っていた。
My grandmother's personal document box contained many old letters.
Compound of 手 (hand, personal) and 文庫 (library, archive), suggesting a small, hand-held repository for documents.