From Rijeka to Pula there are plenty of daily bus departure year around, the travel time by bus is about 2.5 to 3 hours. Some buses drive along the Istrian coast other via the Istrian Ypsilon. If you plan to return to Rijeka from Pula later on, you should consider buying a discounted return ticket, the discount only apply if you travel with SAME bus company both ways, you can see timetable from Pula to Rijeka here. If you are landing a Rijeka airport you first have to take the airport bus to the main bus station, to continue your travel to Pula.