title: Need to be able to download a PDF given a BASE64 String upon componentdidmount
body: <p>I am trying to give the ability to download a PDF given a Base64 string.  I am able to view the PDF using "react-native-view-pdf".  Just unable to figure out how to actually get the file to download.  This is going to need to work for android and ios.</p>
  
  <p>I have tried various forums and am just getting no where sadly.</p>
  
  <p>Note: the this.props.pdf is the Base64 string.  </p>
  
  <p>Attempt 1)</p>
  
  <pre><code>var path = RNFetchBlob.fs.dirs.DocumentDir + "/bill.pdf";
  RNFetchBlob.fs.writeFile(path, this.props.pdf, "base64").then(res =&gt; {
    console.log("File : ", res);
  });
  </code></pre>
  
  <p>Attempt 2) </p>
  
  <pre><code>RNFetchBlob.config({
    fileCache : true,
    appendExt : 'pdf'
  })
  .fetch('GET',`${this.props.PDFLink}`)
  .then((res) =&gt; {
    // open the document directly
    if(Platform.OS == "ios"){
    RNFetchBlob.ios.previewDocument(res.path())
    }
    else{
      RNFetchBlob
      .config({
          addAndroidDownloads : {
              useDownloadManager : true, // &lt;-- this is the only thing required
              // Optional, override notification setting (default to true)
              notification : false,
              // Optional, but recommended since android DownloadManager will fail when
              // the url does not contains a file extension, by default the mime type will be text/plain
              mime : 'application/pdf',
              description : 'File downloaded by download manager.'
          }
      })
      .fetch('GET',`${this.props.PDFLink}`)
      .then((resp) =&gt; {
        // the path of downloaded file
        resp.path()
      })
    }
  })
  .catch(error =&gt; {
    console.error(error);
  });
  </code></pre>
  
  <p>I am expecting to see that when the screen loads, that the user can download the PDF. I already have it displaying to the user, just want them to have the ability to download the file as well.</p>