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.

Single Buffer

If you have a single Buffer you can use its toString method that will convert all or part of the binary contents to a string using a specific encoding. It defaults to utf8 if you don't provide a parameter, but I've explicitly set the encoding in this example.

var req = http.request(reqOptions, function(res) {
    ...

    res.on('data', function(chunk) {
        var textChunk = chunk.toString('utf8');
        // process utf8 text chunk
    });
});

Streamed Buffers

If you have streamed buffers like in the question above where the first byte of a multi-byte UTF8-character may be contained in the first Buffer (chunk) and the second byte in the second Buffer then you should use a StringDecoder. :

var StringDecoder = require('string_decoder').StringDecoder;

var req = http.request(reqOptions, function(res) {
    ...
    var decoder = new StringDecoder('utf8');

    res.on('data', function(chunk) {
        var textChunk = decoder.write(chunk);
        // process utf8 text chunk
    });
});

This way bytes of incomplete characters are buffered by the StringDecoder until all required bytes were written to the decoder.

You can do:

var buf = Buffer.from(bufStr, 'utf8');

But this is a bit silly, so another suggestion would be to copy the minimal amount of code out of the called function to allow yourself access to the original buffer. This might be quite easy or fairly difficult depending on the details of that library.