Hello electrobit,
storing verification data in a public blockchain is in general good use case of blockchain. However, I know at least one project which has the exact same aim (saving pdf-files and certificates in blockchain). I forgot the name, but Google will surely have the answer.
When they explained their model, I had the feeling they used "blockchain" only as a marketing buzzword. Especially in these use-cases of blockchain, you have to think about centralization. Has there to be a trusted party? If yes, why should it be trusted and where is the benefit of using a blockchain instead of a centralized database?
When the team explained their reason for using blockchain I was not convinced. I did not see any advantages of their approach compared to digital signatures or the usage of a centralized database.

If this should not be a "just-4-fun"-project, you honestly have to discuss the following questions:
(Don't answer them here.. otherwise its going to be an endless discussion. Just be sure you have an answer to each question which you honestly believe)

1) Why does the whole pdf need to be stored in the blockchain? Wouldn't it be enough to save the signature of the pdf in the blockchain?
2) What are the advantages of your approach compared to digital signatures for pdfs (check adobe)?
3) If it's about storing documents in a decentralized fashion - What is the benefit compared to storj, filecoin, sia?
3) Should it be a standalone blockchain, forked chain or subchain?
4) Should it be an open blockchain, federated or private blockchain?
3.1) If it should be an open blockchain: - Who are the miners/nodes? What is their benefit?
3.2) If it should be a federated/private blockchain: - What's the benefit of using a blockchain in your use case compared to a centralized (open) database?