A bit of history....
PDF used to be generated by a library used locally and from a 3rd party. There were many bugs and once people went to PHP 5.4, much of that library generated so many errors/warnings/notices that one's error_log file was simply filled with errors related to fonts and deprecated features. I.e. the 3rd party didn't maintain the software.
So in V4, cs-cart chose to provide PDF generation as a software service. I don't speak for them so can't address any regional business considerations they may have used in their decision.
But from an operational standpoint, other than maybe once when cartservices.com was down, there have been literally zero issues with generating PDF's. Unfortunately they introduced a bug that compromised some privacy issues and seem to have been very quick to correct it.
it does surprise me that cs-cart always lays silent on these things and then wonders why people get all spun up when a response from them addressing concerns might just cool things down.
I don't work for cs-cart and have simply tried to provide information about history and current implementation. I'm not a lawyer nor a pollicy maker (outside my own domain). So please don't shoot the messenger!
The KB would not provide guidance for how to build the addon. It's not what the KB is for and the product is designed to use the service. So it is in fact "working as designed".