Hier ist ein Vorschlag, bei dem ich eigentlich nur ein Beispiel aus dem
pgfmanual etwas angepasst habe:
\documentclass[margin=10pt]{standalone}
\usepackage{tikz}
\makeatletter
\pgfdeclareshape{document}{
\inheritsavedanchors[from=rectangle] % this is nearly a rectangle
\inheritanchorborder[from=rectangle]
\foreach \a in {%
center,mid,base,north,south,west,east,%
north west,mid west,base west,south west,%
north east,mid east,base east,south east%
}{\inheritanchor[from=rectangle]{\a}}
\backgroundpath{% this is new
%store lower right in xa/ya and upper right in xb/yb
\southwest \pgf@xa=\pgf@x \pgf@ya=\pgf@y
\northeast \pgf@xb=\pgf@x \pgf@yb=\pgf@y
%compute corner of ``flipped page''
\pgf@xc=\pgf@xa \advance\pgf@xc by 5pt% this should be a parameter
\pgf@yc=\pgf@yb \advance\pgf@yc by-5pt
%construct main path
\pgfpathmoveto{\pgfpoint{\pgf@xa}{\pgf@ya}}
\pgfpathlineto{\pgfpoint{\pgf@xa}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@ya}}
\pgfpathclose
%add little corner
\pgfpathmoveto{\pgfpoint{\pgf@xc}{\pgf@yb}}
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}
\pgfpathlineto{\pgfpoint{\pgf@xa}{\pgf@yc}}
}
}
\makeatother
\begin{document}
\begin{tikzpicture}
\node[document,
draw,
shade,top color=orange!80,
inner sep=6pt
]{Remark};
\end{tikzpicture}
\end{document}
