\documentclass[ngerman]{scrartcl}
\usepackage{babel}
\usepackage[ansinew]{inputenc}
\usepackage{hyperref}
\usepackage{blindtext}

% ----------------------------------------------------------------
\makeatletter
\@ifundefined{pdfoutput}{
\newcommand{\PDFremark}[2][]{}%
}{
\newcommand{\PDFremark}[2][]{%
  \pdfstringdef\x@title{#1}%
  \edef\r{\string\r}%
  \pdfstringdef\x@contents{#2}%
  \pdfannot
  width \linewidth
  height \baselineskip
  depth 0pt
  {
    /Subtype /Text
    /T (\x@title)
    /Contents (\x@contents)
    }%
  }
}
\makeatother
% ----------------------------------------------------------------
\begin{document}

\blindtext\PDFremark{Anmerkung}\blindtext
\end{document}
% ----------------------------------------------------------------