Invalid file url or no viewer for mime type 'application/pdf'

i try to open pdf file on my app , but i have error invalid file url or no viewer for mime type ‘application/pdf’
i use document viewer plugin and this my code

 const options : DocumentViewerOptions={
      title :'My PDF'
    };
    this.document.viewDocument('www/assets/pdf1.pdf','application/pdf',options);