What permanent damage can a 28 day pep course of tenofovir (+ fumarate + emtricitabine) do to one's body? also, how effective would the said combination of drugs be in preventing hiv after 68 hours?

Damage is unlikely: It is most effective when started within 72 hours. Nausea, vomiting, and diarrhea are the most likely short-term side effects. Long-term side effects are not expected. Tenofovir may strain the kidneys or liver - usually in those with kidney disease or hepatitis already - but permanent damage is rare. The risks of long-term organ damage are much more likely from the HIV virus than from this drug