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)_.
function (reason) {
          expect(reason instanceof Error).toEqual(true);
        }
      );
      nonIntegerPromise = nonIntegerPromise.then(
        function () {
          throw new Error("shall fail for non-integer pageNumber parameter");
        
        function (reason) {
          expect(reason instanceof Error).toEqual(true);
        }
      );
      nonNumberPromise = nonNumberPromise.then(
        function () {
          throw new Error("shall fail for non-number pageNumber parameter");
        
        function (reason) {
          expect(reason instanceof Error).toEqual(true);
        }
      );
      await Promise.all([
        outOfRangePromise,
        nonIntegerPromise,
        nonNumberPromise,
      ]);
    });
    it("gets page, from /Pages tree with circular reference", async function () {
      const loadingTask = getDocument(
        buildGetDocumentParams("Pages-tree-refs.pdf")
      );