AES-256 in CBC mode with a random 256-bit file key: V 5, revision 6, crypt filter method AESV3. PDF 2.0.
V
AESV3
Aes256 = 0
ISO 32000-2 §7.6.3.3 (Algorithm 1.A), §7.6.4.3.3 (Algorithm 2.A), §7.6.4.4.7-§7.6.4.4.9 (Algorithms 8 to 10).