To open a link annotation (url), you need to implement the listener PDFLayoutListener and in the method OnPDFOpenURI, you can either open the system intent or a custom one...

To open using the system intent:

@Override
public void OnPDFOpenURI(String uri) {
   try {
        Intent intent = new Intent();
        intent.setAction("android.intent.action.VIEW");
        Uri content_url = Uri.parse(uri);
        intent.setData(content_url);
        startActivity(intent);
   } catch(Exception e){ }
}

To open using a custom intent:

@Override
public void OnPDFOpenURI(String uri) {
   try {
      Intent intent = new Intent(this, WebViewAct.class);
      intent.putExtra("URL", uri);
      startActivity(intent);
   } catch(Exception e) { }
}

WebViewAct:

public class WebViewAct extends Activity {

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        WebView mWebView = new WebView(this);
        mWebView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));

        setContentView(mWebView);

        mWebView.getSettings().setJavaScriptEnabled(true);
        mWebView.loadUrl(getIntent().getStringExtra("URL"));
    }
}