You may consider attaching a digital signature as the last line of each document.
Make the last line read "sha3-256: 0feba.....", where that line is the signature of everything in that file before the signature line. This would make them very difficult to tamper with. Especially if you also keep a catalog of the signatures and sizes for all the files you have.
In fact, a similar technique is used when distributing binary packages. A digital "signature" (which is a hash) can be downloaded from the package distributor separately from the package. This way the person who gets the package can produce a hash of that package on their end and verify that the hashes match.
The added benefit of doing this is that hashes can be communicated via unsecured channels. So if there is any need to preserve confidentiality of the documents, the documents with hashes can be sent by secure channels while hashes can be queried from the sender at a later time by any other channel.
Consider the following scenario: you need to send over 10 .docx documents. You generate the hashes for all of them and append a hash to each document. You can then print out the list of document names and their sizes and hashes on 1 piece of paper. You can then store that piece of paper securely. You can generate copies of that piece of paper without worrying about its confidentiality. You can send that list of digital signatures around without worrying about confidentiality. It can effectively act as a receipt for the digital communication which you sent.