Hello,
I replied via email, but here are the tips again in cause you see them here first:
1. Make sure PayPal IPN is enabled under PayPal. com > Profile > My selling tools > Instant payment notifications.
This way PayPal will tell the plugin the order was complete, without connection from paypal, the plugin can't confirm the order.
2. Make sure the file is in the proper format that is pdf 1.5 acrobat 5.0 compatibility.
3. Enabling sandbox in live account does not enable it in live account and vice versa.
4. The IPN history in PayPal (if IPN is enabled) can give you vital information with the call data you can send me to replicate it and see the actual error thrown.
If you are not sure and you are able to share login information, you can reply to my email with login details and I will be happy to sort it out for you.
Only site login, no PayPal login please.
Thanks