Looking at How to download PDF file with Retrofit and Kotlin coroutines?, you can use:

private const val BUFFER_SIZE = 4 * 1024

private fun copyStreamToFile(inputStream: InputStream, outputFile: File) {
    inputStream.use { input ->
        val outputStream = FileOutputStream(outputFile)
        outputStream.use { output ->
            val buffer = ByteArray(BUFFER_SIZE)
            while (true) {
                val byteCount = input.read(buffer)
                if (byteCount < 0) break
                output.write(buffer, 0, byteCount)
            }
            output.flush()
        }
    }
}

or

private fun InputStream.saveToFile(file: String) = use { input ->
    File(file).outputStream().use { output ->
        input.copyTo(output)
    }
}

Also you should create the file.

private fun createFile(context: Context, name: String): File? {
    val storageDir = context.getExternalFilesDir(Environment.DIRECTORY_DOWNLOADS)?.path
    var file = File("$storageDir/$name.pdf")
    return storageDir?.let { file }
}

I found this worked nicely:

fun File.chunkedSequence(chunk: Int): Sequence<ByteArray> {
    val input = this.inputStream().buffered()
    val buffer = ByteArray(chunk)
    return generateSequence {
        val red = input.read(buffer)
        if (red >= 0) buffer.copyOf(red)
        else {
            input.close()
            null
        }
    }
}

Use it like this.

    file.chunkedSequence(CHUNK_SIZE).forEach {
        // Do something with `it`
    }

Not an exact match to your question but this is the question that came up when I was looking to chunk a file into a sequence of byte arrays.