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(promiseA instanceof Promise).toEqual(true);
      expect(promiseA).toBe(promiseB);
      const pageA = await promiseA;
      const pageB = await promiseB;
      expect(pageA instanceof PDFPageProxy).toEqual(true);
      expect(pageA).toBe(pageB);
    });
    it("gets page index", async function () {
      const ref = { num: 17, gen: 0 }; 
      const pageIndex = await pdfDocument.getPageIndex(ref);
      expect(pageIndex).toEqual(1);
    });
    it("gets invalid page index", async function () {
      const ref = { num: 3, gen: 0 }; 
      try {
        await pdfDocument.getPageIndex(ref);
        
        expect(false).toEqual(true);
      
        expect(reason instanceof UnknownErrorException).toEqual(true);
        expect(reason.message).toEqual(
          "The reference does not point to a /Page dictionary."
        );
      }
    });
    it("gets destinations, from /Dests dictionary", async function () {
      const destinations = await pdfDocument.getDestinations();
      expect(destinations).toEqual({
        chapter1: [{ gen: 0, num: 17  { name: "XYZ"  0, 841.89, null],
      });