Hello,
Your project seems to be easy. I really would like to develop the script, that could help you to mark all these pdf files.
However, it's very difficult to say how long it may take to process 150k files. It depends how many levels your folder tree has, how many pages files have, etc.
Anyway, I'd be honoured if I could try to help you.
My rate is extremely low, because I'm new on this website. You may take advantage of this situation and save some money ;)
Regards,
P.