Thanks to @Alenanno: I tried the solution he suggested and it worked like a charm!
I slightly modified his code, adding a \watermarkon
macro and now watermarks can be easily enabled/disabled for every single page.
This is my code:
\documentclass[12pt,a4paper]{exam}
\usepackage{draftwatermark}
\usepackage{etoolbox}
%%%% for the 1 page %%%%
\usepackage[final]{pdfpages}
%%%%%%%%%%%%%%%%
\usepackage{blindtext}
\SetWatermarkAngle{45}
\SetWatermarkLightness{.8}
\SetWatermarkFontSize{10cm}
\SetWatermarkScale{8}
\SetWatermarkText{\tt{DRAFT}}
%%% Macro to disable watermark
\makeatletter
\def\watermarkoff{%
\@sc@wm@stampfalse
}
\makeatother
%%% Macro to enable watermark
\makeatletter
\def\watermarkon{%
\@sc@wm@stamptrue
}
\makeatother
\begin{document}
%%% no watermark on the first page
\watermarkoff
%%%% for the 1 page %%%%
\includepdf[pages={1},offset=0 0, delta=0 0, scale=1]{./lshort}
%%%%%%%%%%%%%%%%%%
%%% watermark on the second one
\watermarkon
\Blindtext
\newpage
%%% no watermark on the third page
\watermarkoff
\Blindtext
\newpage
%%% watermark on the remaining pages
\watermarkon
\Blindtext
\newpage
\Blindtext
\end{document}
And this is the result:
The first page (lshort) is the cover page of The (Not So) Short Introduction to LaTeX2e.