I checked the logs and I'm seeing Runtime error: exited workflow with code 139 again, as well as FATAL ERROR: MarkCompactCollector: young object promotion failed Allocation failed - JavaScript heap out of memory deeper into the stack trace.
When I've looked up the 139 error internally it seems like looping over large amounts of data is a common culprit, so this could be caused by the step you mentioned where it converts PDFs to images. For these types of errors we recommend either limiting the data that comes in or split the work up into separate workflows.
I've also heard that in this situation, having a webhook response block attached will cause logs not to show up. If you want to see the logs with a more descriptive error you can try to intentionally send a large amount of data through the workflow without the response block attached, and your global error handler should be able to pick it up.
We are hosting Office Hours tomorrow from 2pm - 3pm EDT and that is the best way to get 1 on 1 help from us. I'll be there as well as other knowledgeable folks on the team!