Acid, such as vinegar, lime or lemon juice - but you didn't say if they were supply lines or drains. I'm guessing supply because most pvc drain pipe is below ground (at least in my area).
If drains, same thing - acid of some sort, but if the scale is real bad you're looking at routing I'd say, if not complete replacement. Likely the line diameter is too small for routing.
There is a product called CLR (Calcium Lime Rust if I recall correctly) but not sure that it's meant for potable water lines.
Your problem could extend all the way back to the well pressure line? If you have a pressure tank, likely that's got scale in it too.