As @pete31 notes - it would be very helpful for you to post an example PDF.
But putting aside the potential for AI to detect white space or the potential of AI to detect a large font suggesting a new article, what if you simply had an easy to use low-tech app in which a human marked the transition between two articles?
Using a back of napkin calculation if a staffer can review one periodical per minute with such software, that means 500 periodicals per day or 200 days for the whole task. Allowing for error in the calculations, that’s clearly under 1 year for a full-time clerical person. Surely that does not cost $100,000.
But assuming this is some sort of project of academic or historic merit, you could probably do it even more efficiently by inviting a bunch of college students to help support the endeavor in return for pizza while they pull a few all-nighters as a group to get it done.
One way or another if these periodicals are worthwhile to organize, surely you can get help both to devise an app to split the articles and for the human labor to divide them.
Alternatively - hire a computer science guru student to utilize Amazon Mechanical Turk or some open-source equivalent to outsource the splitting of articles to the web at large. It’s a perfect task for such a project and there are a surprising number of people on the web who either for free or for a nominal amount of money would do such a task. 10 cents per periodical would probably be a generous rate to pay and would get the job done for $10K plus the cost of a techical person to get the process rolling.