okay, is it a linux server or a windoze box?
is the server also your mail server or is that a seperate box?
it's not really possible to be notified IMMEDIATELY, unless you have an account on the mail server.
Assuming it's a linux box, you would probably use procmail (http://www.procmail.org) to process your mail. procmail is a mail filter program. as soon as a mail arrives for you, the server gives it to procmail. you could set up procmail to call your script on the email if it is from paypal.
your script would read the email from stdin, use a regular expression (http://de2.php.net/manual/en/ref.regex.php) to find the relevant filename.
get yourself a mime email class from http://www.phpclasses.org to build an email with the file attached and send it.
if you don't have a shell account on the mail server, you won't be able to set up an IMMEDIATE response - you will need a script which is run every x minutes to check for new mail, probably from cron, if you are using a linux server.