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(loadingTask instanceof PDFDocumentLoadingTask).toEqual(true);
      try {
        await loadingTask.promise;
        
        expect(false).toEqual(true);
      
        expect(reason instanceof InvalidPDFException).toEqual(true);
        expect(reason.message).toEqual("Invalid PDF structure.");
      }
      await loadingTask.destroy();
    });
    it("creates pdf doc from PDF file with inaccessible /Pages tree", async function () {
      const loadingTask = getDocument(
        buildGetDocumentParams("poppler-395-0-fuzzed.pdf")
      );
      expect(loadingTask instanceof PDFDocumentLoadingTask).toEqual(true);
      try {
        await loadingTask.promise;
        
        expect(false).toEqual(true);
      
        expect(reason instanceof InvalidPDFException).toEqual(true);
        expect(reason.message).toEqual("Invalid Root reference.");
      }
      await loadingTask.destroy();
    });
    it("creates pdf doc from PDF files, with bad /Pages tree /Count", async function () {
      const loadingTask1 = getDocument(
        buildGetDocumentParams("poppler-67295-0.pdf")
      );