The problem you face with that kind of process is that you don’t initially know how many files will be processed. Even if you mass-copy all PDFs into a folder at once, the process will have time to start before all the PDFs have actually been written to that folder (the PDFs don’t all simultaneously appear in the folder, there is a delay between the first and the last file).
Therefore, the way I would approach it is to have the self-replicating process create a dummy text file (e.g. “completed.txt”) in the same folder where your PDFs are originally stored. That file will be overwritten as each instance of the self-replicating process runs.
You would then have a different process that starts with a File Count input. It should monitor that same folder and it should have the following extension mask (treated as a regular expression):
which means it will count the total number of PDF and TXT files. If that count is equal to exactly 1, then it means only the TXT file remains in that folder and you can then kick off your output creation process. The nice thing about the File Count input is that if the condition isn’t met, then the process doesn’t run needlessly.
Make sure to delete the TXT file immediately after you have validated that is is the only remaining file in the folder, so that the File Count task doesn’t find it again on its next run.
Using this method, there is an off chance that the last remaining PDF has not finished processing when the output creation process launches. One way to mitigate this issue is to look at the timestamp of the TXT file (the Folder Listing task would allow you to get that time stamp). You could delay the launch of the process until the file is at least, say, 2 minutes old (depending on how long it takes to process a single PDF file).
Not overly complex to implement, but it still requires a bit of work. But that’s to be expected when working with a mix of parallel and sequential processes.
That said, perhaps others users have dealt with this kind of requirement in a different way, so it would be interesting to learn how they went about it.