Specify the MIME type (application/pdf) to allow uploading the PDF file.

JavaScript Code:

var match= ["image/jpeg","image/png","image/jpg", "application/pdf"];
if(!((imagefile==match[0]) || (imagefile==match[1]) || (imagefile==match[2]) || (imagefile==match[3]))){
    alert('Please select a valid image file (JPEG/JPG/PNG/PDF).');
    $("#file").val('');
    return false;
}

PHP Code:

$valid_extensions = array("jpeg", "jpg", "png", "pdf");

if((($_FILES["hard_file"]["type"] == "image/png") || ($_FILES["file"]["type"] == "image/jpg") || ($_FILES["file"]["type"] == "image/jpeg")) || ($_FILES["file"]["type"] == "application/pdf") && in_array($file_extension, $valid_extensions)){
    //upload code goes here
}