实现起来，稍稍简单，无非就是先下载，然后再打开，记住一点：如果已经下载，就直接打开。具体代码如下：联网下载请更换自己项目中的

public class PdfHttpDownloader {
    private Context context;

    public PdfHttpDownloader(Context context) {
        this.context = context;
    }

    /**
     * 打开pdf
     * pdf:PDF url
     * name:pdf文件的名字
     */
    public void startPdfActivity(String pdf, String name) {
        String terPath = getSDPath() + "/trader/" + name + ".PDF";
        File file = new File(terPath);
        if (file.exists()) {
            Intent intent = getPdfFileIntent(terPath);
            context.startActivity(intent);
        } else {
            downLoadPdf(pdf, name);
        }
    }

    public Intent getPdfFileIntent(String path) {
        Intent i = new Intent(Intent.ACTION_VIEW);
        i.addCategory(Intent.CATEGORY_DEFAULT);
        i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        Uri uri = Uri.fromFile(new File(path));
        i.setDataAndType(uri, "application/pdf");
        return i;
    }

    public void downLoadPdf(final String pdf, final String name) {
        Request request = new Request.Builder()
                .url(pdf)
                .get()
                .build();
        OkHttpClient http = new OkHttpClient();
        http.newCall(request)
                .enqueue(new com.squareup.okhttp.Callback() {
                    @Override
                    public void onFailure(Request request, IOException e) {
                    }

                    @Override
                    public void onResponse(Response response) {
                        InputStream is = null;
                        byte[] buf = new byte[1024];
                        int len = 0;
                        FileOutputStream fos = null;
                        String terPath = null;
                        File file = null;
                        try {
                            is = response.body().byteStream();
                            terPath = getSDPath() + "/trader/" + name + ".PDF";
                            file = new File(terPath);
                            fos = new FileOutputStream(file);
                            while ((len = is.read(buf)) != -1) {
                                fos.write(buf, 0, len);
                            }
                            fos.flush();
                            //下载成功
                            if (file.exists()) {
                                Intent intent = getPdfFileIntent(terPath);
                                context.startActivity(intent);
                            }
                        } catch (Exception e) {
                            e.printStackTrace();
                        } finally {
                            try {
                                if (is != null)
                                    is.close();
                            } catch (IOException e) {
                            }
                            try {
                                if (fos != null)
                                    fos.close();
                            } catch (IOException e) {
                            }
                        }
                    }
                });
    }

    private String getSDPath() {
        File sdDir = null;
        boolean sdCardExist = Environment.getExternalStorageState()
                .equals(Environment.MEDIA_MOUNTED);   //判断sd卡是否存在
        if (sdCardExist) {
            sdDir = Environment.getExternalStorageDirectory();//获取跟目录
        }
        return sdDir.toString();
    }
}