PDFUpload.java

    private static final int PICK_PDF_REQUEST = 123;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        showFileChooser();
    }

    private void showFileChooser() {
        Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
        intent.addCategory(Intent.CATEGORY_OPENABLE);
        intent.setType("application/pdf");
        startActivityForResult(intent, PICK_PDF_REQUEST);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == PICK_PDF_REQUEST && resultCode == RESULT_OK) {
            Uri filePath = data.getData();
            Log.d("PATH", "onActivityResult: " + filePath);

            FirebaseStorage firebaseStorage = FirebaseStorage.getInstance();
            StorageReference pdfRef = firebaseStorage
                    .getReference()
                    .child("pdfs/" + filePath.getLastPathSegment());

            pdfRef.putFile(filePath).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
                @Override
                public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                    Log.d("SUCCESS", "onSuccess: Upload succeed");
                }
            });
        }
    }