It might be an old rule. Back in the day, all score reports had all the scores so the PDF didn't provide any more score info. But, in case something went wrong with the official score report (since that needs to go to the school itself and then matched up with your application), the PDF allows the school to make decisions right away.
However, it could also be intentional to see every single score. I think that is an unethical thing for the school to ask. If you want, you could contact the school and ask them why they want to see the PDF?
For one school (back when they could still see all the scores), my electronic score report failed for some reason (I think I entered the wrong institutional code, oops!). So, the school asked for the PDF instead. This school only required General GRE scores, not the Physics GRE, so I redacted my Physics GRE scores from that PDF. I made the redaction very clear and explained that I was not submitting my Physics GRE score since it was not required. They had no problem with this (I got an offer). So, if you don't want to show all of your test scores, you could consider doing that too. Note that it does come with some risk that if the school really wanted to see all your test scores, then this might hurt you since it might annoy them.