From Venice to Pula, there is buses trough out the year, obviously more in the summer season,where there is heavy tourist traffic between Venice and the Istrian peninsula. The travel distance is about 280 km, and the bus travel time is about 5-6 hours depending on number of stops on the route. Most of the lines you can board at the people mover or the Mestre station if that is more convenient. Make sure to get the exact coordinates of where the bus will wait/stop as both places can be a bit confusing. Here you can see timetable for buses from Venice to Pula (and here you can see timetable for buses in the opposite direction)