\documentclass[12pt,a4paper]{article}
%\documentclass[12pt,a4paper,pdftex]{article}% for PDF output by pdflatex 

\usepackage[verbose]{geometry}
\usepackage{showframe}

%-----------------------------------------------------------
% Uncomment one of the following lines for test.

%\geometry{ width=18cm, marginparwidth=4cm, reversemp}
%\geometry{reset, top=0cm }
%\geometry{text={6in,8in}, top=1.2in, left=2cm, nohead}
%\geometry{hdivide={1cm,,2cm}, vdivide={3cm,8in,}, nohead}
%\geometry{ hdivide={0pt,0.5\w,},vdivide={0pt,0.5\h,},noheadfoot}
%\geometry{margin=2cm, marginpar=4cm,reversemp}
%\geometry{margin=10pt, noheadfoot }
\geometry{ margin=10pt, head=20pt, headsep=10pt, foot=30pt}

%-----------------------------------------------------------

\begin{document}

% \printparam % option for showframe.sty

\def\fox{A quick brown fox jumps over the lazy dog.}
\def\foxes{\fox\ \fox\ \fox\ \fox\ \fox\ \fox\ \fox} 
\def\note{This is a marginal note.}
\def\notes{\note\ \note\ \note\ \note\ \note}
\def\mpi{\marginpar{\footnotesize\notes}}
\def\mpii{\marginpar[\footnotesize$\Rightarrow$ 
This is a \textbf{left-side} 
marginal note. \notes]{\footnotesize$\Leftarrow$\ \notes}}
\makeatletter
\ifGeom@includemp\else\let\mpi\relax\let\mpii\relax\fi
\makeatother
\foxes\vfill\centerline{$\vdots$}\vfill%
\foxes\ \foxes\mpi\ \foxes%
\vfill\centerline{$\vdots$}\vfill\foxes
\clearpage
\foxes\ \foxes\mpii\ \foxes\ \foxes%
\vfill\centerline{$\vdots$}\vfill\foxes
\clearpage
\foxes\ \foxes\mpii\foxes\ \foxes%
\vfill\centerline{$\vdots$}\vfill\foxes
\end{document}