在Android中，可以使用以下代码来解决SSL下载失败的问题：

1. 创建一个自定义的TrustManager：

    private static TrustManager[] getTrustManagers() {
        TrustManager[] trustManagers = null;
        try {
            TrustManagerFactory trustManagerFactory = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
            trustManagerFactory.init((KeyStore) null);
            trustManagers = trustManagerFactory.getTrustManagers();
            if (trustManagers.length != 1 || !(trustManagers[0] instanceof X509TrustManager)) {
                throw new IllegalStateException("Unexpected default trust managers:" + Arrays.toString(trustManagers));
            }
        } catch (NoSuchAlgorithmException | KeyStoreException e) {
            e.printStackTrace();
        }
        return trustManagers;
    }

2. 在下载之前，设置信任所有证书：

    private static void trustAllCertificates() {
        try {
            SSLContext sslContext = SSLContext.getInstance("TLS");
            sslContext.init(null, getTrustManagers(), new SecureRandom());
            HttpsURLConnection.setDefaultSSLSocketFactory(sslContext.getSocketFactory());
            HttpsURLConnection.setDefaultHostnameVerifier((hostname, session) -> true);
        } catch (NoSuchAlgorithmException | KeyManagementException e) {
            e.printStackTrace();
        }
    }

3. 调用trustAllCertificates()方法来信任所有证书：

    trustAllCertificates();

4. 进行下载操作：

    URL url = new URL("https://example.com/file.pdf");
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    InputStream inputStream = connection.getInputStream();
    // 保存文件
    File file = new File("path/to/save/file.pdf");
    FileOutputStream fileOutputStream = new FileOutputStream(file);
    byte[] buffer = new byte[4096];
    int bytesRead;
    while ((bytesRead = inputStream.read(buffer)) != -1) {
        fileOutputStream.write(buffer, 0, bytesRead);
    }
    fileOutputStream.close();
    inputStream.close();

这样，你就可以通过信任所有证书来解决Android SSL下载失败的问题。请注意，这会忽略证书验证，存在一定的安全风险。在生产环境中，建议使用正式的证书和正确的证书验证方式。