PDFlib - PDF Security - Security Recommendations, http://www.pdflib.com/knowledge-base/pdf-security/recommendations/2012年9月26日閲覧, "AES-256 according to PDF 1.7 Adobe Extension Level 3 (Acrobat 9) should be avoided because it contains a weakness in the password checking algorithm which facilitates brute-force attacks against the password. For this reason Acrobat X no longer offers Acrobat 9 encryption for protecting new documents (only for decrypting existing documents). In summary, AES-256 according to PDF 1.7 Adobe Extension Level 8/PDF 2.0 or AES-128 according to PDF 1.6/1.7 should be used, depending on whether or not Acrobat X is available. Passwords should be longer than 6 characters and should contain non-alphabetic characters."
