Hi!
I finally received my approval notice on my I-140 yesterday!
I got an RFE for my I-140 on the 15th Day after I upgraded to PP.
The RFE is pretty silly i.e., to prove that I am a permanent employee.
I indeed send in this letter along with my I-140 package.
However, I think USCIS just randomly issue and RFE in order not to refund the PP processing fees if they can't respond within 15 Days.
I responded to the RFE within 3 days via fax and it was approved 1 day after they received it.
Anyway, thank God that now my I-140 journey is over...
and now PATIENTLY waiting for my I-485... 
I finally received my approval notice on my I-140 yesterday!
I got an RFE for my I-140 on the 15th Day after I upgraded to PP.
The RFE is pretty silly i.e., to prove that I am a permanent employee.
I indeed send in this letter along with my I-140 package.
However, I think USCIS just randomly issue and RFE in order not to refund the PP processing fees if they can't respond within 15 Days.
I responded to the RFE within 3 days via fax and it was approved 1 day after they received it.
Anyway, thank God that now my I-140 journey is over...