How to properly step down the voltage of a DC+AC signal that then goes inside a pin of a PIC?
Almost there! :-).
This is a MUCH better attempt than most people make.
For guaranteed voltage levels you want Vpin ideally inside Vdd-Gnd range and at worst not more than 0.3V above Vdd or below ground.  Older data sheets tended to use hard Vdd/gnd limits. More recent ones tend towards a typically 0.3V over/under voltage allowance. Nothing has changed in most cases except a willingness to push the limits more closely with experience of consequences.  Even say 10 uA of current in body diodes CAN produce bizarre results in SOME cases SOMETIMES. 

Place an opposite polarity diode = D4 across D3.
This causes the pin to track Vin - a diode drop in both polarities. 

Maybe make both D4 and D3 Silicon and not Schottky (slightly higher Vf). 

Place two resistors from PIC pin to Vdd and ground. These provide a load when the diodes are driven.

Report back. 

If essential a resistor network on the pin side of D3 D4 can better ensure the pin is never illegally driven BUT what you have plus the above should work well.

This can be done with a few less parts if component count, cost and space are at an utter premium.