No, they probably didn't realize they were tombs at all. They thought the pyramids might contain treasure, and another popular story was that they were granaries built by Joseph/Yusuf in the Bible/Qur'an. There's lots more that could be said about this, but I wrote something about it a few weeks ago: Why did the Sultan of Egypt Al-Aziz Uthman attempt to demolish the Pyramids in the late 12th century?