In the blockchain, only hashes are stored. Moreover, nobody is able to recalculate the original file the hash was calculated from. Thus, the system does not operate with any sensitive data, personal data or classified information. Everybody has to take care of the own data. The hash is calculated from the document locally, the document does not need to be sent. If preferred, the hashes can be completely computed on a separate machine. Hash algorithms are the public open source.
It is possible - but not mandatory - to associate additional meta-data to the hash (reference numbers, bar-codes, names of documents, an arbitrary text and even an address where the document is stored). However, this is an option only. In many applications, the associated meta-data give sense, but in many applications, this might be counterproductive.