She happened to find some old documents in the storage room.
Literal
She [topic-は] storage-room [inside-の中で] by-chance several old-documents [object-を] found.
偶然 ('by chance') is an adverb marking the discovery as accidental. 古文書 ('old documents/manuscripts') is a formal compound. 物置 ('storage room,' literally 'thing-placing-place') is a common household word.