\documentclass[a4paper,12pt]{article}

% To test a layout, uncomment one of lines below.
%------------------------------------------------------------------
% \usepackage{geometry}% uses the default layout.
% \usepackage[width=10cm, left=5cm, vscale=0.8, nohead]{geometry}
% \usepackage[width=17cm, marginparwidth=4cm,reversemp]{geometry}
% \usepackage[hdivide={1cm,,2cm}, vdivide={3cm,8in,}, nohead]{geometry}
% \usepackage[headsep=20pt, head=40pt,foot=40pt]{geometry}
% \usepackage[margin=30pt, head=20pt, twosideshift=20pt]{geometry}
% \usepackage[margin=2cm, marginparwidth=4cm, twoside]{geometry}
% \usepackage[headsep=20pt, head=40pt,foot=20pt]{geometry}
% \usepackage[text={6in,8in}, top=1.2in, left=2cm,nohead]{geometry}
\usepackage[margin=20pt,head=20pt,headsep=20pt,foot=20pt]{geometry}
% \usepackage[margin=2cm,marginpar=3cm,twoside]{geometry}
% \usepackage[margin=2cm,nohead,mag=1414]{geometry}
%------------------------------------------------------------------
\geometry{verbose}% appends verbose option.
\usepackage{showframe}

\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{\raggedright\footnotesize\notes}}
\def\mpii{\marginpar[\raggedright\footnotesize$\Rightarrow$ 
This is a \textbf{left-side} 
marginal note. \notes]{\raggedright\footnotesize$\Leftarrow$\ \notes}}
\makeatletter
\@ifundefined{geometry}{}{%
\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}