首先，将库添加到build.gradle文件中

implementation 'com.github.barteksc:android-pdf-viewer:2.8.2'

要从存储器中打开pdf文件，请使用以下代码。有些评论解释了它的作用。

public class PdfViewActivity2  extends AppCompatActivity {

    // Declare PDFView variable
    private PDFView pdfView;
    private final int PDF_SELECTION_CODE = 99;

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

        // Initialize it
        pdfView = findViewById(R.id.pdfView);

        // Select PDF from storage
        // This code can be used in a button
        Toast.makeText(this, "selectPDF", Toast.LENGTH_LONG).show();
        Intent browseStorage = new Intent(Intent.ACTION_GET_CONTENT);
        browseStorage.setType("application/pdf");
        browseStorage.addCategory(Intent.CATEGORY_OPENABLE);
        startActivityForResult(Intent.createChooser(browseStorage, "Select PDF"), PDF_SELECTION_CODE);
    }

    // Get the Uniform Resource Identifier (Uri) of your data, and receive it as a result.
    // Then, use URI as the pdf source and pass it as a parameter inside this method fromUri(Uri uri)
    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == PDF_SELECTION_CODE && resultCode == Activity.RESULT_OK && data != null) {
            Uri selectedPdfFromStorage = data.getData();
            pdfView.fromUri(selectedPdfFromStorage).defaultPage(0).load();
        }
    }
}

在android 10设备中，你的应用程序无法访问外部存储。
除非你加上

android:requestLegacyExternalStorage="true"

在清单文件的应用程序标记中。

不要使用fromfile（）而是使用fromsource（）。i、 e.将pathe声明为documentsource而不是file。

public class PdfViewActivity2 extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    DocumentSource path = new File(Environment.getExternalStorageDirectory().getPath() + "/Download/myfile.pdf");
    PDFView pdfView = findViewById(R.id.pdfView);
    pdfView.fromSource(path).load();

我已经测试了你的代码，它在android10设备上运行良好。您缺少以下内容：
1.在android清单文件中添加read\u external\u存储权限

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

并且在应用程序标签中添加requestlegacyexternalstorage为true，以便能够访问android 10及以上设备上的外部存储。

<application
        android:requestLegacyExternalStorage="true"

2.验证设备上“/download/pdfs/myfile.pdf”路径下是否存在pdf。
3.通过在运行时首先在oncreate方法中请求外部存储权限，使用以下代码更改活动：

public class PdfViewActivity2 extends AppCompatActivity {

    private static final int READ_STORAGE_PERMISSION_REQUEST_CODE = 1000;

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

        //check if Read External Storage permission was granded
        boolean granded = checkPermissionForReadExtertalStorage();
        if(!granded){
            requestPermissionForReadExtertalStorage();
        }
        else {
           readPdf();
        }
    }

    public boolean checkPermissionForReadExtertalStorage() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            int result = checkSelfPermission(Manifest.permission.READ_EXTERNAL_STORAGE);
            return result == PackageManager.PERMISSION_GRANTED;
        }
        return false;
    }

    public void requestPermissionForReadExtertalStorage() {
        try {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, READ_STORAGE_PERMISSION_REQUEST_CODE);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        switch (requestCode) {
            case READ_STORAGE_PERMISSION_REQUEST_CODE: {
                // If request is cancelled, the result arrays are empty.
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    // permission was granted. Read Pdf from External Storage
                    readPdf();
                } else {
                    // permission denied. Disable the functionality that depends on this permission.
                }
            }
        }
    }

    private void readPdf(){
        File path = new File(Environment.getExternalStorageDirectory().getPath() + "/Download/Pdfs/myfile.pdf");
        PDFView pdfView = findViewById(R.id.pdfView);
        pdfView.fromFile(path).load();
    }
}