public void readText() {
        final TextView textView = (TextView) findViewById(R.id.textView);
        final File directory = Environment.getExternalStorageDirectory();
        final StringBuffer sb = new StringBuffer();
        Flowable
                .create(new FlowableOnSubscribe<String>() {
                    @Override
                    public void subscribe(FlowableEmitter<String> e) throws Exception {
                        try {
                            FileReader reader = new FileReader(directory + "/haha.pdf");
                            BufferedReader br = new BufferedReader(reader);
                            String str;
                            while ((str = br.readLine()) != null && !e.isCancelled()) {
                                while (e.requested() == 0) {
                                    if (e.isCancelled()) {
                                        break;
                                    }
                                }
                                e.onNext(str);
                            }

                            br.close();
                            reader.close();
                            e.onComplete();
                        } catch (Exception err) {
                            e.onError(err);
                        }
                    }
                }, BackpressureStrategy.ERROR)
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<String>() {
                    @Override
                    public void onSubscribe(Subscription s) {
                        mSubscription = s;
                        s.request(1);
                    }

                    @Override
                    public void onNext(String s) {
                        sb.append(s);
                        mSubscription.request(1);
                    }

                    @Override
                    public void onError(Throwable t) {
                    }

                    @Override
                    public void onComplete() {
                        textView.setText(sb);
                    }
                });
    }