From pdf.js/examples/helloworld/hello.js, modify it to open display the page specified at the width of browser. We do this by looking at the width of the PDF's page and the width of the window and then setting the scale of the page's viewport accordingly (see line 5).

open-pdf-with-width-of-screen.js

  PDFJS.getDocument(url).then(function(pdf) {
    // Using promise to fetch the page
    pdf.getPage(1).then(function(page) {
      // Scale the PDF to the width of the window
      var scale = window.innerWidth / page.view[2]
      var viewport = page.getViewport(scale);

      //
      // Prepare canvas using PDF page dimensions
      //
      var canvas = document.getElementById('the-canvas');
      var context = canvas.getContext('2d');
      canvas.height = viewport.height;
      canvas.width = viewport.width;

      //
      // Render PDF page into canvas context
      //
      var renderContext = {
        canvasContext: context,
        viewport: viewport
      };
      page.render(renderContext);
    });
  });