What is the LEGO EV3 rpf file? I read the LEGO Mindstorms EV3 Communication Developer Kit's document. 

I have a question in 4.2.1 (Start program “Demo” on EV3 brick) it is mentioned "The file-extension is "rpf" and NOT "rbf", so how can I generate the rpf file?
rpf stands for "rudolph program file" [source] (I believe "Rudolf" is the internal code name for EV3.)

These are the files created using the "Brick Program" utility on the EV3 brick. Once you have created a program on the brick, you can upload the file using the EV3 desktop software. You can find the files in the BrickProg_Save folder.



However, after explaining all of that, I think the documentation you linked may be a bit misleading. The file extension does not have to be .rpf. The file just has to be an EV3 bytecode file, so a .rbf file could be used too. They probably just chose that example since the Demo.rpf file is installed by default on the EV3.