\documentclass{article}
\usepackage{docassembly}
\usepackage{lipsum}

\hypersetup{pdfpagemode=UseNone}

% Look in the Adobe Acrobat JavaScript manual under the addWatermarkFromText to see all the possible parameters.
\begin{docassembly}
\addWatermarkFromText({
    cText: "Confidential",
    aColor: color.red,
    nFontSize: 72,
    cFont:font.HelvB,
    bOnTop:false,
    bOnPrint:false,
    nOpacity: 0.2,
    nRotation: 45
});
\executeSave();
\end{docassembly}

\begin{document}

\lipsum[1-30]

\end{document} 