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):

["mouseenter", "Mouse Enter"],
            ["mouseleave", "Mouse Exit"],
            ["mouseup", "Mouse Up"],
          ],
          event => event.target.value
        );
      }
      if (blurListener) {
        element.addEventListener("blur", blurListener);
      }
      if (this.data.comb) {
        const fieldWidth = this.data.rect[2] - this.data.rect[0];
        const combWidth = fieldWidth / maxLen;
        element.classList.add("comb");
        element.style.letterSpacing = `calc(${combWidth}px * var(--scale-factor) - 1ch)`;
      }
    } else {
      element = document.createElement("div");
      element.textContent = this.data.fieldValue;
      element.style.verticalAlign = "middle";
      element.style.display = "table-cell";
    }
    this._setTextStyle(element);
    this._setBackgroundColor(element);
    this._setDefaultPropertiesFromJS(element);
    this.container.append(element);
    return this.container;
  }
}
class SignatureWidgetAnnotationElement extends WidgetAnnotationElement {