That's why you cannot edit H.264 in Premiere Pro smoothly. Generally speaking, the H.264 is a delivery codec and it isn't an ideal codec for editing. It can be found in lots of places, which means you may get the H.264 video files from a Camera like GoPro Hero4, Nikon D810, Canon 5D Mark III, or a video sharing site such as YouTube and Fox, or shared from others.
As a quite hot video compression format, H.264 is widely for the recording, compression, and distribution of video content.
