I've tested the stopjob command, and it does appear to work in PlanetPress Image. I've made a loop to display a page 100 times, but after the 10th page, it throws a stopjob to stop the job. The result is a 10 pages pdf, as expected.
With that said, I am not sure how this command would behave in a more complex document. This command forces the job to stop, no matter what it was doing. This may work fine with a simple PressTalk loop, but if there's a more complex PressTalk, many objects and layouts, it might not be a safe command to use to "break" in the middle of processing.
I would suggest to avoid using stopjob() and try a different approach to break. For example, you could simply put a condition on your page that will check whatever it needs to check to see if it's the last page or not, and then have the condition be false if it's beyond the last page. This way, you'd obtain a similar result, but you won't force the job to stop.
RaphaŽl Lalonde Lefebvre