Hi! Mine are trained for both (because the winters can be rough and we both work). But in the summer, they prefer to go outside, and we like that too.
I have heard of moving the pad closer to the door gradually. But you can also let them train themselves by giving "better treats" and a much bigger party (praise) when they go outside. They're smart enough to know what gets them the best deal.
Our older one actually goes to the stairway (his pad is downstairs) and by just looking at us, asks which direction to go. We say "outside" and he dashes to the left. Or we say "downstairs" and he runs down the stairs.
They are very very smart, so if you can find the right way, they can really train themselves and you!