Sets the encryption mode.
As the side effect, ups the version of PDF to 1.4 when the mode is set to PDFDocument.EncryptionModeEnum.ENCRYPT_R3.
PDFDocument.EncryptionModeEnum.ENCRYPT_R2 - Use Revision 2 algorithm. keyLength automatically set to 5 (40 bits).
PDFDocument.EncryptionModeEnum.ENCRYPT_R3 - Use Revision 3 algorithm. keyLength can be 5 (40 bits) to 16 (128bits).