documentclass{article}
usepackage{lipsum}
usepackage{tikz}
usetikzlibrary{fadings,shadings}
usepackage{eso-pic}
usepackage{lastpage,refcount}
deffinaltrans{0.5} % adjust this value to make the end more or less transparent
begin{document}

newcounter{lastpagenumber}%
setcounter{lastpagenumber}{getrefbykeydefault{LastPage}{page}{1}}%
AddToShipoutPictureFG{% Add picture to background of every page
  AtPageLowerLeft{%
        makebox[paperwidth]{
        begin{tikzpicture}
        pgfmathsetmacro{opaone}{100*(1-finaltrans*(thepage)/(thelastpagenumber+1))}
        pgfmathsetmacro{opatwo}{100*(1-finaltrans*(thepage+1)/(thelastpagenumber+1))}
        tikzfading[name=fade down thepage,
            top color=transparent!opaone,
            bottom color=transparent!opatwo]
        fill[white,path fading=fade down thepage] (0,0) rectangle (paperwidth,paperheight);
        end{tikzpicture}}%
  }%
}


lipsum[1-20]
end{document}