Use one of the below methods to carry out the installation safely.Create a bootable copy of the Yosemite OS X installer you have downloaded to a USB driver.
HOW TO DOWNLOAD MAC OS X YOSEMITE ISO INSTALL
Do not proceed with the installation instead, quit the installer by selecting Quit Install OS X from the Install OS X menu. Once the download is complete, the OS X Yosemite Install app will launch on its own.I don't really know what to do with this, but you can have it. A Yosemite App Store DMG converted to ISO format.