Sorry for my late answer, but we have many requests this days...
Here's my tutorial to send signed invoices to your customers:
1.) Sign up at http://www.smskaufen.com/ (They provide many services, PDF signing also)
2.) Create this folders:
whmcs/invoices/pdf/
whmcs/invoices/signed/
3.) Unpack attachment and edit files (<---)
4.) Copy all files from attachment to
whmcs/invoices/
5.) Edit whmcs/includes/actionhooks.php
function actionhook_InvoicePaid($vars) {
# This function runs when an invoice is fully paid and therefore the services renewed
# $vars["InvoiceID"]
$ch = curl_init('https://example.com/whmcs/invoices/hook_paid.php?id='.$vars["InvoiceID"]);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_HEADER, TRUE);
$rueck = curl_exec($ch);
curl_close($ch);
}
6.) Check everything. - If you get an payment and set an invoice to "paid" an signed invoice will automatically send to your customer.
signit.zip