Now imagine using that kind of technology on other day to day transactions from buying and selling of assets (real estate, stocks) to the government's activities it will give you more transparency on your deals and their actions. Someone will input/execute the transaction and everybody can look it up in the blockchain. Just think of it as an tamper proof online ledger where once you successfully inputted a data it can never be change or altered inside the chain.
OP wanted to store a PDF, at $1000 (today) to $100,000 (last December) per MB, that's rediculously expensive to do.
What you can do, is just store the hash of the file. This proves the file existed at the moment the hash was stored, and can't be altered anymore.
Dooglus did this to store Bustabit's hash
here.
I think this is solid answer, IPFS is one of the examples.
Its great that it works even today and you can see great community there.