Typeset Arial Narrow and sans serif mathematical symbols

This also applies to fonts other than Arial Narrow.

1. Use XeLaTeX instead of pdftex.
2. Suppose the desired font for mathematical symbols is Computer Modern Bright:

\usepackage{cmbright}
\usepackage[T1]{fontenc}
\usepackage[no-math]{fontspec} \setsansfont[Ligatures=TeX]{Arial Narrow}

3. Use \sffamily after \begin{document}.

Note: the other way to set font is

\fontspec{Arial Narrow}
\setmainfont{Arial Narrow}