在App内实现直接预览pdf文件，而不是通过调用第三方软件，如WPS office等打开pdf。

主要思路：通过PhotoView将pdf读取为图片流进行展示。

一、首先，获取对本地文件读取的权限

在AndrooidManifest.xml中声明权限，以及页面中动态获取权限。

二、在build.gradle中引入需要的包

    implementation 'com.github.chrisbanes:PhotoView:2.0.0'

三、在项目库地址中添加

以及：

四、添加页面布局文件

添加一个页面布局文件：activity_pdf.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout  xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <android.support.v4.view.ViewPager
        android:id="@+id/vp_pdf"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
    </android.support.v4.view.ViewPager>

</LinearLayout >

添加一个适配器布局：item_pdf.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.github.chrisbanes.photoview.PhotoView
        android:id="@+id/iv_pdf"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </com.github.chrisbanes.photoview.PhotoView>


</LinearLayout>

五、打开和关闭的实现代码

public class PdfActivity extends AppCompatActivity {

    LayoutInflater mInflater;
    PdfRenderer mRenderer;
    ViewPager vpPdf;
    ParcelFileDescriptor mDescriptor;

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

        init();
    }

    public void init() {
        mInflater = LayoutInflater.from(this);
        vpPdf =findViewById(R.id.vp_pdf);

        try {
            openRender();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    private void openRender() throws IOException {
    
        File file = new File(DataHelper.GetHelpPdfPath());

        //初始化PdfRender
        mDescriptor = ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
        if (mDescriptor != null) {
            mRenderer = new PdfRenderer(mDescriptor);
        }

        //初始化ViewPager的适配器并绑定
        MyAdapter adapter = new MyAdapter();
        vpPdf.setAdapter(adapter);
    }

    class MyAdapter extends PagerAdapter {

        @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
        @Override
        public int getCount() {
            return mRenderer.getPageCount();
        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
            return view==object;
        }

        @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            View view = mInflater.inflate(R.layout.item_pdf, null);

            PhotoView pvPdf = view.findViewById(R.id.iv_pdf);
            pvPdf.setEnabled(true);

            if (getCount() <= position) {
                return view;
            }

            PdfRenderer.Page currentPage = mRenderer.openPage(position);
            Bitmap bitmap = Bitmap.createBitmap(1080, 1760, Bitmap.Config.ARGB_8888);
            currentPage.render(bitmap, null, null, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY);
            pvPdf.setImageBitmap(bitmap);
            //关闭当前Page对象
            currentPage.close();

            container.addView(view);
            return view;
        }

        @Override
        public void destroyItem(ViewGroup container, int position, Object object) {
            //销毁需要销毁的视图
            container.removeView((View) object);
        }
    }
//关闭pdf
    @Override
    protected void onDestroy() {
        super.onDestroy();
        try{
            closeRenderer();
        }
        catch (Exception ex){
            ex.printStackTrace();
        }
    }
    //关闭pdf
    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    private void closeRenderer() throws IOException {
        if (mRenderer != null){
            mRenderer.close();
        }
        if (mDescriptor != null){
            mDescriptor.close();
        }
    }

}