\documentclass{beamer}
\geometry{paperwidth=200mm,paperheight=200mm, top=0in, bottom=0.2in, left=0.2in, right=0.2in}

\providecommand{\includemaindocument}{\input{Document.tex}}% fallback definition
\includemaindocument

%\input{../Document.tex}
%"\newcommand\includemaindocument{\input{../<documentfilename>.tex}}\input{<filename>.tex}"