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 page1 = loadingTask.promise.then(function (pdfDoc) {
        return pdfDoc.getPage(1).then(
          function (pdfPage) {
            expect(pdfPage instanceof PDFPageProxy).toEqual(true);
            expect(pdfPage.ref).toEqual({ num: 6, gen: 0 });
          
          function (reason) {
            throw new Error("shall not fail for valid page");
          }
        );
      });
      const page2 = loadingTask.promise.then(function (pdfDoc) {
        return pdfDoc.getPage(2).then(
          function (pdfPage) {
            throw new Error("shall fail for invalid page");
          
          function (reason) {
            expect(reason instanceof UnknownErrorException).toEqual(true);
            expect(reason.message).toEqual(
              "Pages tree contains circular reference."
            );
          }
        );
      });
      await Promise.all([page1, page2]);
      await loadingTask.destroy();
    });
    it("gets page multiple time, with working caches", async function () {
      const promiseA = pdfDocument.getPage(1);
      const promiseB = pdfDocument.getPage(1);