\usepackage{geometry}
\geometry{paperwidth=297mm,paperheight=420mm} % A3

\pdfpageattr{/MediaBox [0 0 841 1190]
             /TrimBox  [0 0 841 1190]}
%/CropBox [] /BleedBox [] %%% not necessary to set
\pdfoptionpdfminorversion=3 % depends on pdftex version %% MUSS 3 SEIN !!!!
\pdfcatalog{
  /OutputIntents [ <<
  /Info (none)
  /Type /OutputIntent
  /S /GTS_PDFX
  /OutputConditionIdentifier (OFCOM_PO_P1_F60_95)
  /RegistryName (http://www.color.org/)
  >> ]
}