initForm() {
    if (!this._font) {
      throw new Error('Must set a font before calling initForm method');
    }
    this._acroform = {
      fonts: {},
      defaultFont: this._font.name,
    };
    this._acroform.fonts[this._font.id] = this._font.ref();

    let data = {
      Fields: [],
      NeedAppearances: true,
      DA: new String(`/${this._font.id} 0 Tf 0 g`),
      DR: {
        Font: {},
      },
    };
    data.DR.Font[this._font.id] = this._font.ref();
    const AcroForm = this.ref(data);
    this._root.data.AcroForm = AcroForm;
    return this;
  }
Must call if adding AcroForms to a document. Must also call font() before
this method to set the default font.