A .docXfile is not a UTF-8 string (it's a binary ZIP file) so when you read it into a Buffer object and then call .toString() on it, you're assuming it is already encoding as UTF-8 in the buffer and you want to now move it into a Javascript string. That's not what you have. Your binary data will likely encounter things that are invalid in UTF-8 and those will be discarded or coerced into valid UTF-8, causing an irreversible change.

What Buffer.toString() does is take a Buffer that is ALREADY encoded in UTF-8 and puts it into a Javascript string. See this comment in the doc,

If encoding is 'utf8' and a byte sequence in the input is not valid UTF-8, then each invalid byte is replaced with the replacement character U+FFFD.

So, the code you show in your question is wrongly assuming that Buffer.toString() takes binary data and reversibly encodes it as a UTF8 string. That is not what it does and that's why it doesn't do what you are expecting.

Your question doesn't describe what you're actually trying to accomplish. If you want to do something useful with the .docX file, you probably need to actually parse it from it's binary ZIP file form into the actual components of the file in their appropriate format.

Now that you explain you're trying to store it in localStorage, then you need to encode the binary into a string format. One such popular option is Base64 though it isn't super efficient (size wise), but it is better than many others. See Binary Data in JSON String. Something better than Base64 for prior discussion on this topic. Ignore the notes about compression in that other answer because your data is already ZIP compressed.

The official node.js documentation for Buffer is the best place to check for something like this. As previously noted, Buffer currently supports these encodings: 'ascii', 'utf8', 'utf16le'/'ucs2', 'base64', 'base64url', 'latin1'/'binary', and 'hex'.

I think the important part of the documentation you linked is When decoding a Buffer into a string that does not exclusively contain valid UTF-8 data, the Unicode replacement character U+FFFD � will be used to represent those errors. When you are converting your buffer into a utf8 string, not all characters are valid utf8, as you can see by doing a console.log(bufferString); almost all of it comes out as gibberish. Therefore you are irretrievably losing data when converting from the buffer into a utf8 string and you can't get that lost data back when converting back into the buffer.

In your example if you were to use utf16 instead of utf8 you don't lose information and thus your buffer is the same once converting back. I.E

const intArr = [31, 139, 8, 0, 0, 0, 0, 0, 0, 0, 170, 86, 42, 201, 207, 78, 205, 83, 178, 82, 178, 76, 78, 53, 179, 72, 74, 51, 215, 53, 54, 51, 51, 211, 53, 49, 78, 50, 210, 77, 74, 49, 182, 208, 53, 52, 178, 180, 72, 75, 76, 52, 75, 180, 76, 50, 81, 170, 5, 0, 0, 0, 255, 255, 3, 0, 29, 73, 93, 151, 48, 0, 0, 0]
const buffer = Buffer.from(intArr);
const bufferString = buffer.toString('utf16le');
const differentBuffer = Buffer.from(bufferString, 'utf16le') ;
console.log(buffer); // same as the below log
console.log(differentBuffer); // same as the above log