\documentclass{article}

\usepackage[a4paper]{geometry}
\usepackage{showframe}

%\geometry{ width=18cm, marginparwidth=4cm, reversemp, verbose}
%\geometry{reset, top=0cm }
%\geometry{text={6in,8in}, top=1.2in, 
%             left=2cm, nohead, verbose}
%\geometry{ hdivide={1cm,*,2cm}, vdivide={3cm,8in,} nohead}
\geometry{ verbose, margin=10pt, noheadfoot }

\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}}

% Comment out the following two lines to appear the maginal notes.
\let\mpi\relax
\let\mpii\relax

\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}