documentclass[a4paper,12pt,twoside]{article}
usepackage[utf8]{inputenc}
usepackage[british]{babel}
usepackage[T1]{fontenc}
usepackage{graphicx}
usepackage{eso-pic,picture}
usepackage{xcolor}
definecolor{gray4}{rgb}{0.4,0.4,0.4}
definecolor{gray8}{rgb}{0.8,0.8,0.8}

makeatletter
AddToShipoutPicture{%
  AtPageLowerLeft{%
    hspace*{0.48cm}rotatebox{90}{%
      colorbox{gray8!44}{%
        begin{minipage}{paperheight}
          sffamily
          vspace*{0.4em}
          centering
          hspace*{stretch{1}}
          textcolor{gray4}{some vertical text here along the pages}
          hspace*{stretch{1}}
          vspace*{0.4em}
        end{minipage}%
      }%
    }%
  }%
}%
makeatother

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
begin{document}

newpage{}
section{Intro}
newpage{}
section{Methods}
newpage{}

end{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%