verify pdf signature with openssl

in pdf signature, the "/contents" includes digest value of the pdf file and the enc_digest 256 bytes. decrypt the enc_digest, the output is not same with the digest value. but recalculate the digest of the pdf bytes in byterange, it is correct digest. so I guess the ppklite not only use the digest to encrypt, but also other attributes. what is "other attributes"? does anyone know about this? thank you very much!