How to decrypt an encrypted file correctly?

I use the cryptography library: androidx.security:security-crypto:1.0.0-beta01 according to this example https://developer.android.com/guide/topics/security/cryptography I cannot correctly decrypt the file I encrypted. After decryption, all pages of the pdf file are blank. the screenshot shows the file size changes. Can anyone tell me what I'm doing wrong?

object Encryptor {

private lateinit var context: Context
private val keyGenParameterSpec = MasterKeys.AES256_GCM_SPEC
private val masterKeyAlias = MasterKeys.getOrCreate(keyGenParameterSpec)

fun decrypt(file: File): File {
    val bytes = file.encrypted.openFileInput().readBytes()
    return File("${context.filesDir}/storage", "decrypt.pdf").apply { writeBytes(bytes) }
}

fun encrypt(fromFile: File, toFile: File) {
    val text = fromFile.readText()
    toFile.encrypted.openFileOutput().bufferedWriter().use { bufferWriter ->
        bufferWriter.write(text)
    }
}

private val File.encrypted: EncryptedFile
    get() = EncryptedFile.Builder(
        this,
        context,
        masterKeyAlias,
        EncryptedFile.FileEncryptionScheme.AES256_GCM_HKDF_4KB
    ).build()
}

The code snippet from the example decrypts the file for a very long time (6 sec +/-) And this code also does not solve my problem

val contents = file.encrypted.openFileInput().bufferedReader().useLines { lines ->
        lines.fold("") { working, line ->
            "$working\n$line"
        }
    }
someFile.bufferedWriter().write(contents)