\documentclass{scrartcl}
\usepackage[no-math]{fontspec}
\usepackage{xunicode}
\usepackage{xltxtra}
\usepackage[svgnames]{xcolor}
\usepackage{tikz}
%\usepackage[unicode]{hyperref}
\usepackage[subject={Top1},author={Ånsgar Lund}]{pdfcomment}
\setmainfont[Mapping=tex-text]{Bergamo}
\begin{document}
\fontspec[SmallCapsFont={BergamoSmc},]{Bergamo}
\definecolor{MyBlue}{rgb}{0.045,0.278,0.643}
A \pdfcomment[subject={Top2},author={Daisy Duck},color={0.234 0.867 0.211}]{This is a comment.} little Test! \pdfcomment[color=MyBlue,icon=Note,open=true,hspace=100pt]{This is another comment.} test the hspace! \pdfcomment[color=Gold,icon=Star,author={Captain Jack}]{It's enough now!}

\colorlet{KernFarbe}{red}
\colorlet{HuelleFarbe}{blue}
\vspace{35mm}
\begin{tikzpicture}[
        Huelle/.style = {ball color=HuelleFarbe!25},
        Kern/.style = {ball color=KernFarbe!25},
        Proton/.style = {ball color=HuelleFarbe!80},
        Elektron/.style = {ball color=KernFarbe!80},
    ]

    \shade[Huelle] (8,0) circle (2cm);
    \shade[Kern]   (8,0) circle (.7cm);

    % Elektronen
    \foreach \cx/\cy in {8/0,8/.3,8/-.3,8.2/.25,8.2/-.25,%
        8.3/.09,8.3/-.09,7.8/.25,7.8/.-.25,7.7/.09,%
        7.7/-.09,7.5/.09,8.5/-.09,7.6/-.3,8/-.5,8/.5,8.4/.4}{
            \shade[Elektron] (\cx,\cy) circle (.6mm);
    }
    % Protonen
    \foreach \cx/\cy in {6.9/-.1,7.4/1.2,7/.7,7.9/1.5,9.2/0.2,%
        9/1,7.5/-1.2,7/-0.8,9/-0.8,8.7/-1.2,7.7/-1.5,8.9/.5, %8.6,-1
        7/1.3,6.6/.4,9.6/-.31,9.6/.6,8.3/1.2}{
            \shade[Proton] (\cx,\cy) circle (.6mm);
    }
    
    \draw[<-] (8.25,.55) -- +(1.4,1.4) node[above] {Atomkern};
    \draw[<-] (7,.4) -- +(-1.3,1) node[above] {Atomhülle};
    \draw[<-] (6.8,-.085) -- +(-1.6,.4) node[above] {Elektron};
    \draw[<-] (7.5,-.35) -- +(-2,-.7) node[below] {Proton};
\end{tikzpicture}
\pdfcomment[color=lime,subject={Top2},icon=Check,author={Dr. Albertus Newton}]{Wow, that's nice!}\pdfcommentsetup{color=NavyBlue,subject={Top1},icon=Circle,author={Prof. Dolittle}}\pdfmargincomment{Yeah nice, but stolen from the pgf-Users mailing list!}

\vspace{2cm}
For a long time \texttt{pdflatex} offers the command \texttt{\textbackslash pdfannot} for inserting arbitrary pdf annotations. However, in a form where additional knowledge of the pdf reference is indispensable. \texttt{pdfcomment.sty} is an answer to the - from time to time - emerging questions in newsgroups, how one could use the comment function of \texttt{Adobe Reader}. At least out of the \LaTeX{} code \texttt{pdfcomment.sty} offers a convenient and user-friendly possibility to use pdf annotations in pdf files. \pdfmargincomment{Holy moly! Can anybody transfer that to real english!}

Unfortunately the support of pdf annotations by pdf viewers is only partly available to nonexistent. The reference viewer for the development of this package is \texttt{Adobe Reader}.
\end{document}