\documentclass[12pt]{article}
  \usepackage[T1]{fontenc}
  \usepackage[utf8]{inputenc}

\usepackage{varioref}
\usepackage{xr-hyper}
\usepackage[colorlinks]{hyperref}
\usepackage{cleveref}
\newtheorem{thm}{Theorem}
\newtheorem{defn}{Definition}
\newtheorem{lem}{Lemma}
\newtheorem{cor}{Corollary}
\newtheorem{prop}{Propostion}

\usepackage{equation}
\externaldocument{file1}

\begin{document}

This is a test
(\ref{E:EULERPOISSON})
\end{document}