private document; personal letter
Refers to a document or letter that is personal or confidential, as opposed to official or public correspondence.
I can't show this private letter to anyone else.
While organizing my personal documents, I found an old letter.
post office box; P.O. box
Compound of 私 (private) and 書 (document/writing). The exact historical derivation is uncertain, but the spelling is conventionally associated with personal or non-official documents.