PDCP buffers SDUs also. Until discard timer expires or it gets acknowledged.
In lossless handover pending downlink packets (i.e. PDCP SDUs for which no Acks are received from UE, PDCP SDU which were not transmitted and new GTP packets that are coming from S1 interface in source eNB )are sent to the target eNB.
Control plane in source eNB sends the 'Next Transmit Seq num' to target eNB.This information is passed to the the target eNB via SN-STATUS-TRANSFER..
The Status Transfer message provides the sequence number (SN) and the Hyper Frame Number (HFN) that the target eNodeB should assign to the first packet with no SN yet assigned that it must deliver. This first packet can
either be one received over the target S1 path or one received over X2, if data forwarding over X2
I hope it will help you...
Let me know if you have any doubt.