Look at here:
1) Most of the land battle scenes from PR maybe shot by Saban themselves (if they're able to borrow clothes from Toei Company) or borrowed the scenes (One time I was watching PR SPD, I've noticed Jasmine and Umeko's names while their changing to SWAT mode)
2) MOST of the Zord/Mecha battles were simply borrowed from the original, since (I think) shooting a mecha fight scene would be hard, why not borrow from the original.
3) the storyline of sentai ones was WAAAYYY TOO DIFFERENT from PR, since Saban have to devise a script for American audiences.
4) Sentai is for all ages, PR definitely focuses on the kid audiences.