Can PVC schedule 40 marked ridgid, be run 6" underground just like steel ridgid?

Table 300.5 Underground Installations,(A) Minimum Cover Requirements, should have what you need to know.

No you can not, the code refers to PVC as RNC or Rigid Nonmetallic Conduit but that does not mean the rules of RMC (Rigid Metal Coduit) apply.

As zapped said, Table 300.5 gives the info which varies depth depending on location.

Under traffic 24" (except dwellings 18")

Generally 18"

Bellow 2" cement slab 12"

Bellow 4" cement slab 4"

Under a building 0"