从sdcard中选择一个pdf，在android studio中将其转换为字节数组。

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

private static final int REQUEST_CODE_PICK_PDF = 1;

private void pickPdfFromSdCard() {
    Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
    intent.setType("application/pdf");
    startActivityForResult(intent, REQUEST_CODE_PICK_PDF);
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == REQUEST_CODE_PICK_PDF && resultCode == RESULT_OK) {
        Uri uri = data.getData();
        convertPdfToByteArray(uri);
    }
}

private void convertPdfToByteArray(Uri uri) {
    try {
        InputStream inputStream = getContentResolver().openInputStream(uri);
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();

        byte[] buffer = new byte[1024];
        int length;
        while ((length = inputStream.read(buffer)) != -1) {
            byteArrayOutputStream.write(buffer, 0, length);
        }

        byte[] byteArray = byteArrayOutputStream.toByteArray();
        // 在这里可以使用字节数组进行进一步的处理

        inputStream.close();
        byteArrayOutputStream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}