How to insert a pdf as background?

You will need the package eso-pic (eso-pic package)

\usepackage{eso-pic}

Before \begin{document}, define:

\newcommand\BackgroundPic{ \put(0,0){ \parbox[b][\paperheight]{\paperwidth}{% \vfill \centering \includegraphics[width=\paperwidth,height=\paperheight]{BackGroundPDF.pdf} \vfill }}}

After \begin{document}:

\AddToShipoutPicture*{\BackgroundPic} \maketitle

the * states that the background picture is only used on that page.

I recently encountered this problem and I found the solution from https://latex.org/forum/viewtopic.php?t=26829 works better for me. Since this question here pops out on the top when I search for it. It may be beneficial to also have the code here.

It uses the package background.

\documentclass[10pt]{letter}
\usepackage{background}
\backgroundsetup{scale = 1, angle = 0, opacity = 0.2,
   contents = {\includegraphics[width = \paperwidth,
   height = \paperheight, keepaspectratio]
   {Background.pdf}}}
\begin{document}
Main text.
\end{document}