Sorry I'm busy at the moment so haven't had a chance to have a proper look at the diagram but the PIC inputs need about 0.25 x VDD for a TTL input and about 0.8 x VDD for a schmitt trigger input.
It's a pity you can't view the output of your optical sensor with an oscilloscope.
Edit: Just saw your other post. Is that line the output of your last op-amp stage or is that the output from your sensor? Edit again: Scratch that, your current buffer should be fine.
Perhaps you need a comparator after the last stage of your signal buffer to square up the signal? Edit again: Scratch that, your current buffer should be fine.