\documentclass[a4paper]{article}
%\VignetteIndexEntry{Computation of variance-covariance matrix}
%\VignettePackage{Countr} 
\usepackage{pdfpages}

\def\pdffile{./var_covar.pdf}

\begin{document}

\IfFileExists{\pdffile}{%
  \includepdf[fitpaper=true,pages=-]{\pdffile}}%
{\typeout{No file \pdffile}}%

\end{document}