Pdf Downloaded As Invalid Format In React native using RNFetchBlob

Question

I am trying to download a pdf from the URL. I am using RNFetchBlob to download pdf from URLs. It is also getting downloaded successfully. But it doesn't open. It says invalid format. At first, I thought it might be because I was running in an emulator but I tried on a real device also it is also giving an invalid format.

Below is the code I am using:

 const downloadFile = async (url, title) => {
        const grantedstorage = await PermissionsAndroid.request(
            PermissionsAndroid.PERMISSIONS.WRITE_EXTERNAL_STORAGE);
        if (grantedstorage === PermissionsAndroid.RESULTS.GRANTED) {

            const { dirs } = RNFetchBlob.fs;
            const dirToSave = Platform.OS == 'ios' ? dirs.DocumentDir : dirs.DownloadDir
            const configfb = {
                fileCache: true,
                addAndroidDownloads: {
                    useDownloadManager: true,
                    notification: true,
                    mediaScannable: true,
                    title: title + ".pdf",
                    path: dirToSave + "/" + title + ".pdf",
                    mime : 'application/pdf',
                }
            }

            const configOptions = Platform.select({
                ios: {
                    fileCache: configfb.fileCache,
                    title: configfb.title,
                    path: configfb.path,
                    appendExt: 'pdf',
                    notification: configfb.notification
                },
                android: configfb,
            });



            RNFetchBlob.config(configOptions)
                .fetch('GET', url,{})
                .then((res) => {
                    if (Platform.OS === "ios") {
                        RNFetchBlob.fs.writeFile(configfb.path, res.data, 'base64');
                        RNFetchBlob.ios.previewDocument(configfb.path);
                    }
                    if (Platform.OS == 'android') {
                        alert('File downloaded');
                    }
                    console.log('The file saved to ', res.path());
                })
                .catch((e) => {

                    console.log('Catch ERROR', e.message)
                });
        } else if (grantedstorage === PermissionsAndroid.RESULTS.DENIED) {
            alert("Please allow permission to storage if you want to download file.");
        }
        else {
            alert("Please go to app setting and allow permission to storage.");
        }
    }

Please help me solve this. And yes the URL is of a protected pdf if that is of any concern, please let me know.