When merging PDF files using itext the result pdf show 0 bytes in android studio - pdf

Question

My code is here; I was selecting PDF files from a SD card or internal phone memory using the showFileChooser() method then from onActivityResult(), the files go to mergePdfFiles(View view) method, and from there the files go to createPdf(String[] srcs).
Here is the complete code of my activity:
//Below is onCreate();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_mergedpdf);
adapter = new ArrayAdapter<>(this,
android.R.layout.simple_list_item_1, locations);
// public void fileSelected(File file ) {
// locations.add(file.toString());
// adapter.notifyDataSetChanged();
// }
NewText = (TextView)findViewById(R.id.textView);
AdView mAdView = findViewById(R.id.adView);
AdRequest adRequest = new AdRequest.Builder().build();
mAdView.loadAd(adRequest);
listView = (ListView)findViewById(R.id.list_items);
btn = (ImageView) findViewById(R.id.imageView8);
btnconvert = (ImageView) findViewById(R.id.imageView11);
btnconvert.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// btTag=((Button)v).getTag().toString();
try {
createPdf( locations);
} catch (DocumentException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
//Toast.makeText(Mergedpdf.this, "button clicked", Toast.LENGTH_SHORT).show();
}
});
btn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// btTag=((Button)v).getTag().toString();
showFileChooser();
NewText.setText("Below Files are Selected");
}
});
}
//this is mergedfiles();
public void mergePdfFiles(View view){
Toast.makeText(Mergedpdf.this, "merge function", Toast.LENGTH_SHORT).show();
try {String[] srcs= new String[locations.size()];
for(int i = 0;i<locations.size();i++) {
srcs[i] = locations.get(i);
}
// String[] srcs = {txt1.getText().toString(), txt2.getText().toString()};
createPdf(srcs);
}catch (Exception e){e.printStackTrace();}
}
//This method create merged pdf file
public void createPdf (String[] srcs) {
try {
// Create document object
File docsFolder = new File(Environment.getExternalStorageDirectory() + "/Merged-PDFFiles");
if (!docsFolder.exists()) {
docsFolder.mkdir();
Log.i(TAG, "Created a new directory for PDF");
}
Date date = new Date();
#SuppressLint("SimpleDateFormat") final String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(date);
Document document = new Document();
// pdfCopy = new File(docsFolder.getAbsolutePath(),pdf+"Images.pdf");
// Document document = new Document();
//PdfWriter.getInstance(document, output);
// Create pdf copy object to copy current document to the output mergedresult file
PdfCopy copy = new PdfCopy(document, new FileOutputStream(docsFolder + "/" + timeStamp +"combine.pdf"));
Toast.makeText(Mergedpdf.this, "merged pdf saved", Toast.LENGTH_SHORT).show();
// Open the document
document.open();
PdfReader pr;
int n;
for (int i = 0; i < srcs.length; i++) {
// Create pdf reader object to read each input pdf file
pr = new PdfReader(srcs[i].toString());
// Get the number of pages of the pdf file
n = pr.getNumberOfPages();
for (int page = 1; page <= n; page++) {
// Import all pages from the file to PdfCopy
copy.addPage(copy.getImportedPage(pr, page));
}
}
document.close(); // close the document
} catch (Exception e) {
e.printStackTrace();
}
}
//below is showfilechooser(); method
private void showFileChooser () {
Log.e("AA", "bttag=" + btTag);
String folderPath = Environment.getExternalStorageDirectory() + "/";
Intent intent = new Intent();
intent.setAction(Intent.ACTION_GET_CONTENT);
Uri myUri = Uri.parse(folderPath);
intent.setDataAndType(myUri, "application/pdf");
Intent intentChooser = Intent.createChooser(intent, "Select a file");
startActivityForResult(intentChooser,PICKFILE_RESULT_CODE);
}
//below is onActivityResult(); method
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
if (data != null) {
if (requestCode == PICKFILE_RESULT_CODE) {
if (resultCode == RESULT_OK) {
String FilePath = data.getData().getPath();
locations.add(FilePath);
ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,R.layout.listview,locations);
listView.setAdapter(adapter);
}
}
}
}
//And my declared variables before oncreate().
public com.itextpdf.text.Document Document;
public PdfCopy Copy;
public ByteArrayOutputStream ms;
TextView NewText;
private TextView txt1;
private Button bt1, bt2,bt3;
private Handler handler;
ListView listView;
ArrayList<String> locations = new ArrayList<>();
ArrayAdapter<String> adapter;
ImageView btn, btnconvert, btn3;
private static final String TAG = "PdfCreator";
private final int PICKFILE_RESULT_CODE=10;
private String btTag = "";