How to send a second email after paypal payment?

I'm using Zen Cart 139H with PayPal IPN (Basic PayPal service).

I'm selling information, so the user who buys something will receive a PDF file, that has a password on it.
Also I'm giving free PDFs that password are not required.

My issue is that when user buy a PDF and pay using paypal, he/she should receive the password for the PDF immediately, but I don't know how to do it.
I know how to change the paypal payment received email, but how to attached a password specific for that PDF the user bought?

The process is like this:

1. user go to the zen cart and 'buy' a free pdf.
2. The link is imedialty send to the user and he/she can donwload it.

or

1. the user go and buy a PDF type A
2. I have to manually check for it, and send to that user the password for the PDF A

or

1. the user bo and buy a PDF type B
2. I have to manually check for it, and send to that user the password for the PDF A

Is there any way I can automatic send the correct password for the PDF the user bought?

I'm only using Paypal as payment.

Thank you.

First, you will give your customers better experience if you use the PayPal Express Checkout module instead of the IPN module.

Second, you can trigger additional content to appear in the emails that the store sends, by intercepting the order process with custom code. Much like adding a software serial number to an order. This works easily when your customer is purchasing an item that they'll download from your site. ie: a downloadable product, not something you email to them after they buy.

So, set your products up as downloadable items, and your customers will receive a download link automatically.

Next, add your own custom code to issue a password/pin/serialnumber/whatever when the order email is generated.
Best to hook into the NOTIFY_ORDER_PROCESSING_ATTRIBUTES_EXIST notifier point, but there are other ways you could tackle the same thing.
There are several threads discussing this topic already, including these: www.zen-cart.com/forum/search.php?do=process&query=$custom_insertable_text plus others.