Many OEM's approve PPAP electronically now in their own portals & not by hard signature. You can't hold a gun to their head to force out signatures .
Always best to check with your customer . We were rold b7y IATF that if you make due dilligence and request with emails , & phone calls , that if they answer by phone agree that PPAP is ok'd then record the name & title of that person , date & that could serve as due dilligence in cases where electronic approval is not tne norm . Quality systems need to catch up with technology.
Communications also need to be improved with technology . A call to your customer purchasing / SQA can go along way . If they answer the phone or retrun the call...