Not able to save blob to file using @ionic-native/file writeFile() method

i’m using ionic file to save blob which i’m getting in response from an api, response is in form of string which is a blob.

Trying to convert blob(which is in string format) to ArrayBuffer then saving ArrayBuffer to file.writeFile() method.

    public saveAsPdfFile(buffer: any, fileName: string): void {
            let successMsg:any;
            let data = this.str2ab(buffer);
            //this.translate.get('downloadedSuccessfully').subscribe((res: string) => { successMsg = res; });
            // atob(this.string_Base64PDF)
            this.file.createFile(url, fileName+".pdf", true)
                .then(() => {
                    console.log("file created");
                    this.file.writeFile(url, fileName+".pdf", data,{ replace: true }).then(() => {
                      }).catch((err) => {

     str2ab(str) {
        var buf = new ArrayBuffer(str.length*2); // 2 bytes for each char
        var bufView = new Uint16Array(buf);
        for (var i=0, strLen=str.length; i < strLen; i++) {
          bufView[i] = str.charCodeAt(i);
        return buf;

file is getting created but when i open error says “filename.pdf is of invalid format” tried same with excel also thows same error. let me know what i’m missing