Since you have GNU sed you can install GNU awk. With GNU awk for multi-char RS and RT:

    $ awk -v RS='href="http[^"]+.pdf"' -F'"' 'RT{$0=RT; print $2}' file
    https://ecole-euclid.cnrs.fr/wp-content/uploads/EDE2019_Henrot-Versillé-C1_L1.pdf
    https://ecole-euclid.cnrs.fr/wp-content/uploads/EDE2019_Henrot-Versillé_C1_L2.pdf
    https://ecole-euclid.cnrs.fr/wp-content/uploads/EDE2018_Henrot-Versillé_C3.pdf
    https://ecole-euclid.cnrs.fr/wp-content/uploads/EDE2018_Martinelli_C2_L1_Bayesian.pdf
    https://ecole-euclid.cnrs.fr/wp-content/uploads/EDE2018_Martinelli_C2_TD_Bayesian.pdf