The invention provides a PDF digital signature method. The PDF digital signature method includes the steps that byte range data corresponding to a digital signature are calculated according to a PDF file to be signed and attribute information of the digital signature; an abstract value is calculated according to the byte range data, and a countersignature digital signature result is calculated according to the abstract value; a main digital signature result is calculated according to the abstract value; the countersignature digital signature result is packaged in the main digital signature result, and the digital signature result obtained after packaging is stored in a PDF file digital signature dictionary. Correspondingly, the invention further provides a PDF digital signature system and a PDF digital signature verification method and system. By means of the PDF digital signature method and system and the PDF digital signature verification method and system in the technical scheme, the same version of the PDF file can be subjected to digital signing for multiple times, and the shared signature of the content of the same file is achieved; as the countersignature signature result of a user-defined algorithm is packaged into the main signature result of a standard algorithm, the compatibility between the PDF file signed with the user-defined algorithm and a universal PDF reader is achieved.