There is already a PDF uploaded to Monero, removing tx_extra will make this more expensive, but it won't stop this, nothing will. Monero at its core is just a data protocol.
I just hope the futile attempts at "banning" stenography won't compromise other aspects of the protocol.
Also, why are u excited for storing data on a ledger that's:
1 - Doomed to collapse by its own emission schedule.
2 - Prone to censorship.
