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)_.
beforeAll(function () {
    CanvasFactory = new DefaultCanvasFactory();
  });
  afterAll(function () {
    CanvasFactory = null;
  });
  function waitSome(callback) {
    const WAIT_TIMEOUT = 10;
    setTimeout(function () {
      callback();
     WAIT_TIMEOUT);
  }
  function mergeText(items) {
    return items.map(chunk => chunk.str + (chunk.hasEOL ? "\n" : "")).join("");
  }
  describe("getDocument", function () {
    it("creates pdf doc from URL-string", async function () {
      const urlStr = TEST_PDFS_PATH + basicApiFileName;
      const loadingTask = getDocument(urlStr);
      expect(loadingTask instanceof PDFDocumentLoadingTask).toEqual(true);
      const pdfDocument = await loadingTask.promise;
      expect(typeof urlStr).toEqual("string");
      expect(pdfDocument instanceof PDFDocumentProxy).toEqual(true);
      expect(pdfDocument.numPages).toEqual(3);
      await loadingTask.destroy();
    });
    it("creates pdf doc from URL-object", async function () {
      if (isNodeJS) {
        pending("window.location is not supported in Node.js.");
      }