Javascript - Previewing PDFs During Upload

Use-Cases of this Tutorial

  • You are looking to know how to show a preview of the PDF before it is uploaded to server.
  • You are looking to know a basic usage of the PDF.JS library.

The very useful PDF.JS library makes it possible to show a preview of the PDF before uploading to server.


Loading Preview ..

Download codes for demo


  • PDF.JS files are included in HTML code.
  • <input type="file" /> will accept a PDF file from the user.
  • Validation is performed to check whether file is a valid PDF and size has not exceeded allowed limit.
  • URL.createObjectURL method creates an object url from the chosen PDF.
  • PDF.JS library is initialized taking the object url as the source url of the PDF.
  • PDF.JS' APIs getDocument and getPage are used to render the PDF. The first page of the PDF is rendered as an image, and that is shown as the preview of the PDF.

PDF.JS is an awesome library. As a detailed preview, you can even show all pages of the PDF with the "Previous Page" / "Next Page" navigation buttons. For a complete tutorial check PDF.JS Tutorial - How to display a PDF with Javascript.

PDF.JS files :

<script src="pdf.js"></script>
<script src="pdf.worker.js"></script>


<button id="upload-dialog">Choose PDF</button>
<input type="file" id="pdf-file" accept="application/pdf" style="display:none" />
<div id="pdf-loader" style="display:none">Loading Preview ..</div>
<canvas id="pdf-preview" width="150" style="display:none"></canvas>

Javascript :

// will hold the PDF handle returned by PDF.JS API
var _PDF_DOC;

// PDF.JS renders PDF in a <canvas> element
var _CANVAS = document.querySelector('#pdf-preview');

// will hold object url of chosen PDF

// load the PDF
function showPDF(pdf_url) {
    PDFJS.getDocument({ url: pdf_url }).then(function(pdf_doc) {
        _PDF_DOC = pdf_doc;

        // show the first page of PDF

        // destroy previous object url
    }).catch(function(error) {
        // error reason

// show page of PDF
function showPage(page_no) {
    _PDF_DOC.getPage(page_no).then(function(page) {
        // set the scale of viewport
        var scale_required = _CANVAS.width / page.getViewport(1).width;

        // get viewport of the page at required scale
        var viewport = page.getViewport(scale_required);

        // set canvas height
        _CANVAS.height = viewport.height;

        var renderContext = {
            canvasContext: _CANVAS.getContext('2d'),
            viewport: viewport
        // render the page contents in the canvas
        page.render(renderContext).then(function() {
            document.querySelector("#pdf-preview").style.display = 'inline-block';
            document.querySelector("#pdf-loader").style.display = 'none';

/* showing upload file dialog */
document.querySelector("#upload-dialog").addEventListener('click', function() {

/* when users selects a file */
document.querySelector("#pdf-file").addEventListener('change', function() {
    // user selected PDF
    var file = this.files[0];

    // allowed MIME types
    var mime_types = [ 'application/pdf' ];
    // validate whether PDF
    if(mime_types.indexOf(file.type) == -1) {
        alert('Error : Incorrect file type');

    // validate file size
    if(file.size > 10*1024*1024) {
        alert('Error : Exceeded size 10MB');

    // validation is successful

    // hide upload dialog
    document.querySelector("#upload-dialog").style.display = 'none';
    // show the PDF preview loader
    document.querySelector("#pdf-loader").style.display = 'inline-block';

    // object url of PDF 
    _OBJECT_URL = URL.createObjectURL(file)

    // send the object url of the pdf to the PDF preview function

Final Upload to Server

Javascript - AJAX File Uploading is a tutorial on file uploading to server.

Javascript - Previewing Images During Upload