As of September 1, 2018, the China National Intellectual Property Administration (CNIPA – the new name for the combined China Patent & Trademark Office) will no longer automatically retrieve electronic priority documents from the USPTO via the PDX system. Instead, CNIPA will retrieve priority documents via WIPO DAS, which will require providing the DAS access code. The access code is the 4-digit confirmation number listed on USPTO filing receipts and used in the USPTO Electronic Filing System (EFS) and does not need to be requested independently from WIPO.
Note the above applies to applications filed via the Paris Convention. For international (PCT) applications filed with the RO/US, the applicant should check the appropriate item in Box No. VI of the Request (Form PCT/RO/101) to request the RO/US to prepare and transmit to the International Bureau a certified copy of any earlier U.S. application to which priority is claimed (PCT Rule 17.1(b)).
Further, the above does not apply to design patent applications, in which the certified priority documents must be transmitted to CNIPA via other means (e.g., via scanned PDF for e-filing with CNIPA).