\documentclass[a4paper]{article}
%\VignetteIndexEntry{glmmEP User Manual}
%\VignettePackage{glmmEP} 
\usepackage{pdfpages}

\begin{document}

\newread\PDFtoInclude
\openin\PDFtoInclude=glmmEP.pdf
\ifeof\PDFtoInclude
\typeout{No file glmmEP.pdf}
\else
\includepdf[fitpaper=true,pages=-]{glmmEP.pdf}
\fi

\end{document}