Keltec's website states that +p can be used but not for continuous use.
I know Im late to this, but I recently tried sum Winchester Ranger Nato rounds in my Kel-Tec PF9 and I believe those are +P and I havent had an issue. I only ran maybe 2 mags thru it to be safe. Ive heard that you shouldnt continuously feed most pistols with +P ammo because it will wear them down, so I havent done it that much.
"Just like the P-11, the PF-9 will accept +P ammunition, however, not with continuous use." This is on Kel Tec's website in the Pf9's description. Personally, I am not comfortable with using +p ammo in my PF9 because although I love it, it IS cheaply built.