I updated the example from the Pdf.js github http://jsbin.com/pdfjs-prevnext-v2/edit#html,live to scale properly to a fixed canvas width. See http://jsfiddle.net/RREv9/ for my code.
The important line is
var viewport = page.getViewport(canvas.width / page.getViewport(1.0).width);
because the expression canvas.width / page.getViewport(1.0).width gives us the appropriate scaling factor.
You should change the width of your canvas not with css but by the width attribute of the canvas. See Canvas width and height in HTML5