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)_.
});
      
      const loadingTask3 = getDocument(
        buildGetDocumentParams("bad-PageLabels.pdf")
      );
      const promise3 = loadingTask3.promise.then(function (pdfDoc) {
        return pdfDoc.getPageLabels();
      });
      const pageLabels = await Promise.all([
        promise0,
        promise1,
        promise2,
        promise3,
      ]);
      expect(pageLabels[0]).toEqual(["i", "ii", "iii", "1"]);
      expect(pageLabels[1]).toEqual(["Front Page1"]);
      expect(pageLabels[2]).toEqual(["1", "2"]);
      expect(pageLabels[3]).toEqual(["X3"]);
      await Promise.all([
        loadingTask0.destroy(),
        loadingTask1.destroy(),
        loadingTask2.destroy(),
        loadingTask3.destroy(),
      ]);
    });
    it("gets default page layout", async function () {
      const loadingTask = getDocument(
        buildGetDocumentParams("tracemonkey.pdf")
      );
      const pdfDoc = await loadingTask.promise;