[devote]

var re = /(?:[\s\S]*\.pdf|([\s\S]*))$/i;

var lines = [
    'some string',
    'some stringpdf',
    'some string.pdf',
    'some string.PDF'
];

for(var i = 0; i < lines.length; i++) {
    alert(re.exec(lines[i])[1]);
}