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(outlineItem.title).toEqual("hello\x11world");
      await loadingTask.destroy();
    });
    it("gets non-existent permissions", async function () {
      const permissions = await pdfDocument.getPermissions();
      expect(permissions).toEqual(null);
    });
    it("gets permissions", async function () {
      
      const loadingTask0 = getDocument(
        buildGetDocumentParams("issue9972-1.pdf")
      );
      const promise0 = loadingTask0.promise.then(function (pdfDoc) {
        return pdfDoc.getPermissions();
      });
      
      const loadingTask1 = getDocument(
        buildGetDocumentParams("issue9972-2.pdf")
      );
      const promise1 = loadingTask1.promise.then(function (pdfDoc) {
        return pdfDoc.getPermissions();
      });
      
      const loant(
        buildGetDocumentParams("issue9972-3.pdf")
      );
      const promise2 = loadingTask2.promise.then(function (pdfDoc) {
        return pdfDoc.getPermissions();
      });
      const totalPermissionCount = Object.keys(PermissionFlag).length;