Downloading Mac OS X to a PC
No, you can't download OS X installer from Internet, it is available only via Mac App Store.

If your want to build a PC able to run Mac OS X, this project is called OSx86 or also known as Hackintosh.

There are a lot of guides and hardware compatibility lists on tonymacx86 website.
For example, if you want to install Mac OS X Yosemite on an Intel based PC you can follow this instructions. As you can see, you have to previously download the Mac OS X image from the App Store to build a bootable USB drive.