Gets or sets the algorithm. The default is Aes256; Aes256Gcm is its ISO/TS 32003 extension, and the others are deprecated in PDF 2.0.
public PdfEncryptionAlgorithm Algorithm { get; set; }
ISO 32000-2 §7.6.2, Table 20; §7.6.4.2, Table 21; ISO/TS 32003 §5.1.