documentclassではなくて、documentstyleのときはpackageの宣言の仕方が違う
documentstyleのとき
\documentstyle [epsf,12pt,amsmath,amssymb]{article}
documentclassのとき
\documentclass[preprint,eqsecnum,aps,epsf]{revtex4}
\usepackage{amsmath}
\documentstyle [epsf,12pt,amsmath,amssymb]{article}
\documentclass[preprint,eqsecnum,aps,epsf]{revtex4}
\usepackage{amsmath}