% create a new file handle
\newwrite\pdfpcnotesfile

% open file on \begin{document}
\AtBeginDocument{%
    \immediate\openout\pdfpcnotesfile\jobname-Notizen.txt.tmp\relax
    \immediate\write\pdfpcnotesfile{}
}
% define a # https://tex.stackexchange.com/a/37757/10327
\begingroup
    \catcode`\#=12
    \gdef\hashchar{#}%
\endgroup
% define command \pnote{} that works exactly like \note but
% additionally writes notes to file in pdfpc readable format
\newcommand{\spnote}[1]{%
    % keep normal notes working
    \note{#1}%
    % write notes to file
    \begingroup
        \let\#\hashchar
        %\immediate\write\pdfpcnotesfile{\#\theframenumber\#\unexpanded{#1}}%
        \immediate\write\pdfpcnotesfile{\#\thepage\#\unexpanded{#1}}%
    \endgroup
}
% close file on \end{document}
\AtEndDocument{%
    \immediate\closeout\pdfpcnotesfile
}