\documentclass{minimal}

%Use one of the following three lines:
\paperwidth594mm \paperheight841mm %--- activate to get the poster in format DIN A1
%\paperwidth420mm \paperheight594mm %--- activate to get the poster in format DIN A2
%\paperwidth297mm \paperheight420mm %--- activate to get the poster in format DIN A3

\makeatletter
%\@tempdima\paperheight \paperheight\paperwidth \paperwidth\@tempdima %--- activate if and only if you want to scale a poster in landscape format
\makeatother

\usepackage{pdfpages}

\begin{document}
\includepdf{poster.pdf}%--- when indicated, replace "poster.pdf" by the name of your unscaled poster-PDF
\end{document}
