Reading from the source code:

https://github.com/mozilla/pdf.js/blob/57d196e3497e77c81c19f852715ce79d9fb91492/src/display/annotation_layer.js#L2846

it appears to me it was not an intention to mark downloadManager as a required parameter in AnnotationLayerParameters:

https://github.com/mozilla/pdf.js/blob/57d196e3497e77c81c19f852715ce79d9fb91492/src/display/annotation_layer.js#L63

and AnnotationLayerBuilderOptions:

https://github.com/mozilla/pdf.js/blob/57d196e3497e77c81c19f852715ce79d9fb91492/web/annotation_layer_builder.js#L40

and it should be optional, similarly to accessibilityManager for example:

https://github.com/mozilla/pdf.js/blob/57d196e3497e77c81c19f852715ce79d9fb91492/web/text_layer_builder.js#L30

**Configuration:**
- PDF.js version: 2.11.174
- Is a browser extension: no

**What is the expected behavior? (add screenshot)**

`downloadManager` to be optional

**What went wrong? (add screenshot)**

`downloadManager` marked as required

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

selectElement.multiple = true;
          },
          remove(event) {
            const options = selectElement.options;
            const index = event.detail.remove;
            options[index].selected = false;
            selectElement.remove(index);
            if (options.length > 0) {
              const i = Array.prototype.findIndex.call(
                options,
                option => option.selected
              );
              if (i === -1) {
                options[0].selected = true;
              }
            }
            storage.setValue(id, {
              value: getValue( true),
              items: getItems(event),
            });
            selectedValues = getValue( false);
          },
          clear(event) {