The Scots ran out comfortable 123-run victors after posting a solid target of 287 for five.
Kyle Coetzer led the way with the bat as he notched 74 runs, while Richie Berrington chalked up 56. Dylan Budge and Michael Leask were 46 and 50 not out respectively at the end of the 50 overs.
Main stopped Papua New Guinea making any headway in their chase by taking five wickets as they were halted on 164 runs in the 37th over.
The Scots will bid to make it four tour wins out of four tomorrow when they face Oman, whom they overcame in dramatic fashion earlier this week.