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 pdfDocument = await loadingTask.promise;
      expect(pdfDocument.numPages).toEqual(1);
      const page = await pdfDocument.getPage(1);
      expect(page instanceof PDFPageProxy).toEqual(true);
      const opList = await page.getOperatorList();
      expect(opList.fnArray.length).toEqual(0);
      expect(opList.argsArray.length).toEqual(0);
      expect(opList.lastChunk).toEqual(true);
      await loadingTask.destroy();
    });
    it("creates pdf doc from PDF file with bad XRef header", async function () {
      const loadingTask = getDocument(
        buildGetDocumentParams("GHOSTSCRIPT-698804-1-fuzzed.pdf")
      );
      expect(loadingTask instanceof PDFDocumentLoadingTask).toEqual(true);
      const pdfDocument = await loadingTask.promise;
      expect(pdfDocument.numPages).toEqual(1);
      const page = await pdfDocument.getPage(1);
      expect(page instanceof PDFPageProxy).toEqual(true);
      const opList = await page.getOperatorList();
      expect(opList.fnArray.length).toEqual(0);
      expect(opList.argsArray.length).toEqual(0);
      expect(opList.lastChunk).toEqual(true);
      await loadingTask.destroy();
    });
    it("creates pdf doc from PDF file with bad XRef byteWidths", async function () {
      
      const loadingTask = getDocument(
        buildGetDocumentParams("REDHAT-1531897-0.pdf")
      );