Try this:
<?php
if(get_field('pdf_paris')){
$pdfParis = get_field('pdf_paris');
echo'<h2><a href="'.$pdfParis.'" target="_blank">Download <br />the PDF for Paris</a></h2>';
else if(get_field('pdf_marseille')){
$pdfMars = get_field('pdf_marseille');
echo'<a href="'. $pdfMars.'" target="_blank">Download <br />the PDF for Marseille</a>';
else {
echo'No PDF available.';
}
?>
Have fun!