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(page.userUnit).toEqual(1.0);
    });
    it("gets view", function () {
      expect(page.view).toEqual([0, 0, 595.28, 841.89]);
    });
    it("gets view, with empty/invalid bounding boxes", async function () {
      const viewLoadingTask = getDocument(
        buildGetDocumentParams("boundingBox_invalid.pdf")
      );
      const pdfDoc = await viewLoadingTask.promise;
      const numPages = pdfDoc.numPages;
      expect(numPages).toEqual(3);
      const viewPromises = [];
      for (let i = 0; i < numPages; i++) {
        viewPromises[i] = pdfDoc.getPage(i + 1).then(pdfPage => {
          return pdfPage.view;
        });
      }
      const [page1, page2, page3] = await Promise.all(viewPromises);
      expect(page1).toEqual([0, 0, 612, 792]);
      expect(page2).toEqual([0, 0, 800, 600]);
      expect(page3).toEqual([0, 0, 600, 800]);
      await viewLoadingTask.destroy();
    });
    it("gets viewport", function () {
      const viewport = page.getViewport({ scale: 1.5, rotation: 90 });
      expect(viewport.viewBox).toEqual(page.view);
      expect(viewport.scale).toEqual(1.5);
      expect(viewport.rotation).toEqual(90);
      expect(viewport.transform).toEqual([0, 1.5, 1.5, 0, 0, 0]);