As noted above, your code has error. It should be:
email='[email protected]';
$content= "Dear Whoever,
NB: Please click <a href=\"document.pdf\" target=\"_blank\">here</a> to read and download the terms and conditions.";
mail( $email, "Welcome", $content, "From: [email protected]");
You would need to set the mime type to HTML in the header, and use it as a parameter in the mail() function.
From the manual
// To send HTML mail, the Content-type header must be set
$headers = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
// Additional headers
$headers .= 'To: Mary <[email protected]>, Kelly <[email protected]>' . "\r\n";
$headers .= 'From: Birthday Reminder <[email protected]>' . "\r\n";
$headers .= 'Cc: [email protected]' . "\r\n";
$headers .= 'Bcc: [email protected]' . "\r\n";
// Mail it
mail($to, $subject, $message, $headers);
Though I usually use SwiftMailer and it has other neat features.