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(firstImgData.height).toEqual(EXPECTED_HEIGHT);
          expect(firstImgData.kind).toEqual(ImageKind.RGB_24BPP);
          expect(firstImgData.data instanceof Uint8ClampedArray).toEqual(true);
          expect(firstImgData.data.length).toEqual(25245000);
        } else {
          const objsPool = i >= NUM_PAGES_THRESHOLD ? commonObjs : objs;
          const currentImgData = objsPool.get(objId);
          expect(currentImgData.width).toEqual(firstImgData.width);
          expect(currentImgData.height).toEqual(firstImgData.height);
          expect(currentImgData.kind).toEqual(firstImgData.kind);
          expect(currentImgData.data instanceof Uint8ClampedArray).toEqual(
            true
          );
          expect(
            currentImgData.data.every((value, index) => {
              return value === firstImgData.data[index];
            })
          ).toEqual(true);
        }
      }
      await loadingTask.destroy();
      firstImgData = null;
    });
  });
  describe("Multiple `getDocument` instances", function () {
    // Regression test for https://github.com/mozilla/pdf.js/issues/6205
    // A PDF using the Helvetica font.
    const pdf1 = buildGetDoc
    // A PDF using the Times font.
    const pdf2 = buildGetDocumen