From the wget manual at GNU (https://www.gnu.org/software/wget/manual/html_node/Recursive-Accept_002fReject-Options.html):
Specify comma-separated lists of file name suffixes or patterns to accept or reject (see Types of Files). Note that if any of the wildcard characters, ‘’, ‘?’, ‘[’ or ‘]’, appear in an element of acclist or rejlist, it will be treated as a pattern, rather than a suffix. In this case, you have to enclose the pattern into quotes to prevent your shell from expanding it, like in ‘-A ".mp3"’ or ‘-A '*.mp3'’.
So you may try:
wget -P -e robots=off -A pdf -r -l1 http://concours-maths-cpge.fr/