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)_.
);
      arrayPromise = arrayPromise.then(
        function () {
          throw new Error("shall fail for non-string destination.");
        
        function (reason) {
          expect(reason instanceof Error).toEqual(true);
        }
      );
      await Promise.all([numberPromise, booleanPromise, arrayPromise]);
    });
    it("gets non-existent page labels", async function () {
      const pageLabels = await pdfDocument.getPageLabels();
      expect(pageLabels).toEqual(null);
    });
    it("gets page labels", async function () {
      
      const loadingTask0 = getDocument(buildGetDocumentParams("bug793632.pdf"));
      const promise0 = loadingTask0.promise.then(function (pdfDoc) {
        return pdfDoc.getPageLabels();
      });
      
      const loadingTask1 = getDocument(buildGetDocumentParams("issue1453.pdf"));
      const promise1 = loadingTask1.promise.then(function (pdfDoc) {
        return pdfDoc.getPageLabels();
      });
      
      const loant(buildGetDocumentParams("rotation.pdf"));
      const promise2 = loadingTask2.promise.then(function (pdfDoc) {
        return pdfDoc.getPageLabels();