File size is ENORMOUS...Send Help!

This is an 8.5 by 11 doc in illustrator and I know I have two images in there of decent quality, when I save this as a PDF its 550MB. I have evern tried saving in a lower size as well as making sure images werent in as high of quality. This isnt the only illustrator file that this has happened in. Is it because the photos being used start as high quality images? What is happening that my files are taking up this much room? Does illustrator save history every time something happens? Please help!