。
She wore a sad-looking face while reading the letter.
Literal
She [topic-は] that letter [object-を] was-reading [time-とき] sad-looking face [object-を] was-doing.
~顔をする ('make/wear a face') frames the facial expression as something the subject is actively doing — a step livelier than saying 顔だった ('was a face'). Past progressive ていた combined with とき ('when') stretches the action across the reading time: she was wearing this expression throughout. 顔をする is a productive idiom for facial expressions: 嫌な顔をする ('make an unpleasant face'), 困った顔をする ('look troubled'), 知らん顔をする ('feign ignorance').