Try this :-

<input id="inputFile" type="file" onchange="convertToBase64();" />

<script type="text/javascript">
    function convertToBase64() {
        //Read File
        var selectedFile = document.getElementById("inputFile").files;
        //Check File is not Empty
        if (selectedFile.length > 0) {
            // Select the very first file from list
            var fileToLoad = selectedFile[0];
            // FileReader function for read the file.
            var fileReader = new FileReader();
            var base64;
            // Onload of file read the file content
            fileReader.onload = function(fileLoadedEvent) {
                base64 = fileLoadedEvent.target.result;
                // Print data in console
                console.log(base64);
            };
            // Convert data to base64
            fileReader.readAsDataURL(fileToLoad);
        }
    }
</script>

You can just create an Image object and put the base64 as its src, including the data:image... part like this:

var image = new Image();
image.src = 'data:image/png;base64,iVBORw0K...';
document.body.appendChild(image);

It's what they call "Data URIs" and here's the compatibility table for inner peace.

You are not waiting on the promise returned by render() to finish async operation. See https://github.com/mozilla/pdf.js/blob/master/examples/learning/prevnext.html#L76 how to wait on completion:

  ...
  var renderTask = page.render(renderContext);
  // Wait for rendering to finish
  renderTask.promise.then(function () {
    // Page rendered, now take snapshot.
  });
  ...

Please use the below code to convert Base64 to PDF with the client side JavaScript. Pass the base64 data to the function base64ToArrayBuffer

function base64toPDF(data) {
    var bufferArray = base64ToArrayBuffer(data);
    var blobStore = new Blob([bufferArray], { type: "application/pdf" });
    if (window.navigator && window.navigator.msSaveOrOpenBlob) {
        window.navigator.msSaveOrOpenBlob(blobStore);
        return;
    }
    var data = window.URL.createObjectURL(blobStore);
    var link = document.createElement('a');
    document.body.appendChild(link);
    link.href = data;
    link.download = "file.pdf";
    link.click();
    window.URL.revokeObjectURL(data);
    link.remove();
}

function base64ToArrayBuffer(data) {
    var bString = window.atob(data);
    var bLength = bString.length;
    var bytes = new Uint8Array(bLength);
    for (var i = 0; i < bLength; i++) {
        var ascii = bString.charCodeAt(i);
        bytes[i] = ascii;
    }
    return bytes;
};