Java MuPDF集成

我试图集成muPDF，但只显示了一个带有“dismise”按钮的空对话框。 目标是通过单击浮动按钮来显示PDF。我的条件之一是 有什么想法吗，我做错了什么？ 我用“import com.artifex.muPDF.Viewer.DocumentActivity”导入了muPDF查看器，并按照他们主页上的说明安装了muPDF

公共类SingleWFActivity扩展了AppCompatActivity{

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

    Intent singleWF = getIntent(); //sets ID of Workflow
    int editableWfId = singleWF.getIntExtra("Item", 0); //Is needed for getting the right 
    PDF


    FloatingActionButton nextStep = (FloatingActionButton)  
    findViewById(R.id.nextStep_invoice);

    nextStep.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            setNextStep();
            Toast.makeText(SingleWFActivity.this, "Workflow in die nächste Stufe gehoben.", 
            Toast.LENGTH_LONG).show();

            Intent ready = new Intent(SingleWFActivity.this, MainActivity.class);
            startActivity(ready);
            finish();
        }
    });

    FloatingActionButton seePDF = (FloatingActionButton) findViewById(R.id.seePDF);
    seePDF.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Uri documentUri = Uri.parse("http://www.jena.de/fm/41/test.pdf");
            startMuPDFActivity(documentUri);
        }
    });

}


public void startMuPDFActivity(Uri documentUri) {
    Intent intent = new Intent(this, DocumentActivity.class);
    intent.setAction(Intent.ACTION_VIEW);
    intent.setData(documentUri);
    startActivity(intent);
}


public int getActualStep() {
    //TODO: Getting a file, which provides some database like structures for setting up the 
nextStep()
    int stage = 0;
    return stage;
}

public int setNextStep() {

    int actualStage = getActualStep();
    int steps = actualStage;
    steps++;

    return steps;
}


}