Hello!

Let me know if i should have opened this issue on the [pdfjs-dist](https://github.com/mozilla/pdfjs-dist) repo but I though here would fit.

I'm using pdfjs-dist and I'm in need of some more types to be exported. I have no idea how the types end up in the dist-repo but would love to open a PR if only someone could point me in the right direction 😃 Since I was using an older version of pdfjs-dist I realize the only type I need that's not in the type defs in the latest release are _PageViewport_ from _[display/display_utils](https://github.com/mozilla/pdf.js/blob/master/src/display/display_utils.js#L153)_.
expect(viewport.width).toEqual(1262.835);
      expect(viewport.height).toEqual(892.92);
    });
    it('gets viewport with "offsetX/offsetY" arguments', function () {
      const viewport = page.getViewport({
        scale: 1,
        rotation: 0,
        offsetX: 100,
        offsetY: -100,
      });
      expect(viewport.transform).toEqual([1, 0, 0, -1, 100, 741.89]);
    });
    it('gets viewport respecting "dontFlip" argument', function () {
      const scale = 1,
        rotation = 0;
      const viewport = page.getViewport({ scale, rotation });
      const dontFlipViewport = page.getViewport({
        scale,
        rotation,
        dontFlip: true,
      });
      expect(dontFlipViewport).not.toEqual(viewport);
      expect(dontFlipViewport).toEqual(viewport.clone({ dontFlip: true }));
      expect(viewport.transform).toEqual([1, 0, 0, -1, 0, 841.89]);
      expect(dontFlipViewport.transform).toEqual([1, 0, -0, 1, 0, 0]);
    });
    it("gets viewport with invalid rotation", function () {
      expect(function () {
        page.getViewport({ scale: 1, rotation: 45 });
      }).toThrow(