This strange abbreviation has been surfacing in Russian online discussions for some time now.
ППКС (peh-peh-kah-es or pehpex) simply means 'I sign under every word of it' — 'подписываюсь под каждым словом'. Alternatively, it may mean 'I totally agree with what is said' — 'полностью присоединяюсь к сказанному'.
As Lurkmore notes with their usual sarcasm, it is used mostly by people who haven't much to say but are itching to say something. Lurkmore, though not your usual dictionary or Wikipedia, should be recommended as a lively and usually reliable source on modern Russian idioms.
It's not clear how and when exactly the acronym appeared. It may be suggested that it was a side-effect of the popularity of the qualifying IMHO (in my humble opinion, ИМХО in Russian). Someone may have decided to invent an opposite, something short and assertive instead of the wobbly IMHO.