Modal window doesn't appear

Question

I have a problem with angular. I am trying to call a function to generate a pdf by clicking a button in one form. PDF generation is working well because the response i got from the api is correct but in angular the modal windows doesn't appear.

That's the HTML

<div class="pb-5">
  <button (click)="downloadPdf(downloadPdfTemp)" class="btn btn-link pl-0 pt-0">
    <i class="fa fa-plus mr-2"></i> Genera ora il modulo precompilato </button>
</div>
<ng-template #downloadPdfTemp let-c="close" let-d="dismiss" let-modal>
  <div class="modal-header">
    <h4 class="modal-title" id="modal-basic-title-2">Documento generato!</h4>
    <button type="button" class="close" aria-label="Close" (click)="d('Cross click')">
      <span aria-hidden="true">&times;</span>
    </button>
  </div>
  <div class="modal-body">
    <p>Il tuo documento è pronto per essere scaricato e firmato. Clicca sul pulsante in basso.</p>
  </div>
  <div class="modal-footer">
    <a [href]="api + pathNamePdf" class="btn btn-primary" target="_new" (click)="modal.dismiss()">
      <i class="fa fa-download pr-2"></i>Scarica ora </a>
  </div>
</ng-template>

And that's the typescrypt part with the downloadPDF

downloadPdf(content): void {
        this.announcements
            .announcementGenerateDoc(
                this._id,
                this.userId,
                this.announcementObj.additionalInfos
            )
            .pipe(takeUntil(this.destroy$))
            .subscribe(genDoc => {
                this.tmpGenPdf = genDoc.name;
                this.announcements
                    .announcementGeneratePdf(this._id, this.userId)
                    .pipe(takeUntil(this.destroy$))
                    .subscribe(pathName => {
                        this.pathNamePdf = pathName.name;
                        this.modalService.open(content, { centered: true });
                    });
            });
    }

I just tried to add a delay because pdf file is generating like 1-2 second after the button has been pressed but the modal window doesn't appear.

I noticed that the modal windows appears only if i refresh the page.