I think it's a good idea, but isn't it spam in the bitcoin block chain?
You pay so that full nodes stock this data forever, so it is not spam. However it could be seen as inappropriate.
Shouldn't there be a separate blockchain for document verification, so the bitcoin blockchain can be only about transferring money?
Why the Bitcoin block chain is used is because it is the one with the more
chainwork, which makes it the most unlikely to change.
You are utilizing blockchain for which it has been invented.
The block chain has been designed to order transactions.