pdfconfig.tex

\documentclass[12pt]{scrartcl}
\usepackage{pgfkeys}

\begin{document}

\newcommand{\pdfconfigfilename}{FOO}
%\newcommand{\Pdfconfigfilename}{FOO}

\pgfkeys
{
  /includepdffiles/.is family, /includepdffiles,
  % Calling the keys so that the macros are defined.
  default/.style =
  {
    pdfconfigfilename,
  },
  pdfconfigfilename/.default = {XXX},
  pdfconfigfilename/.estore in = \pdfconfigfilename,
}

\NewDocumentCommand{\IncludePDFFiles}{O{}}
{
  \pgfkeys{/includepdffiles, default, #1}%
  \typeout{PDFCONFIGFILENAME in IncludePDFFiles MACRO IS \pdfconfigfilename}
  \typeout{\pdfconfigfilename}
}

\IncludePDFFiles[pdfconfigfilename=\pdfconfigfilename]
%\IncludePDFFiles[pdfconfigfilename=\Pdfconfigfilename]

\end{document}

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