//download del file dal Web
//ATTENZIONE! il file PDF NON viene salvato nella memoria del dispositivo
private fun downloadPdfFromInternet(url: String, dirPath: String, fileName: String) {
    PRDownloader.download(url, dirPath, fileName)
        .build()
        .start(object : OnDownloadListener {
            override fun onDownloadComplete() {
                Toast.makeText(this@MainActivity, "Download completato", Toast.LENGTH_LONG).show()
                val downloadedFile = File(dirPath, fileName)
                showPdfFromFile(downloadedFile)
             }

             override fun onError(error: com.downloader.Error?) {
                Toast.makeText(this@MainActivity, "Errore durante il download del file: $error", Toast.LENGTH_LONG).show()
             }
        })
}