Attach (recommended) or Link to PDF file here: https://www.recom-power.com/pdf/Powerline-AC-DC/RAC05-K.pdf

Configuration:
- Web browser and its version: Firefox 56.0 (64-bit)
- Operating system and its version: Windows 10 with OS Build 15063.632
- PDF.js version: ?
- Is an extension: Built-in

Steps to reproduce the problem:
1. Open Page

What is the expected behavior? To properly render the first page. 
![image](https://user-images.githubusercontent.com/1151969/31415345-b1504022-add7-11e7-855b-8030a308f7a2.png)

What went wrong? Renders poorly.
![image](https://user-images.githubusercontent.com/1151969/31415363-d8842096-add7-11e7-818d-728c1102891b.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension): ?

const { context: ctx } = this.cachedCanvases.getCanvas(
      "isFontSubpixelAAEnabled",
      10,
      10
    );
    ctx.scale(1.5, 1);
    ctx.fillText("I", 0, 10);
    const data = ctx.getImageData(0, 0, 10, 10).data;
    let enabled = false;
    for (let i = 3; i < data.length; i += 4) {
      if (data[i] > 0 && data[i] < 255) {
        enabled = true;
        break;
      }
    }
    return shadow(this, "isFontSubpixelAAEnabled", enabled);
  }
  showText(glyphs) {