薄 has two pronunciations: báo (everyday speech: thin paper, thin clothes) and bó (formal/literary: 薄弱 bóruò weak, 刻薄 kèbó harsh). For HSK, báo is the main one.
usage
The opposite of 薄 is 厚 (hòu, thick). This pair is very common: 薄的书 (thin book) vs 厚的书 (thick book).