How do I uninstall Adobe Reader?

Installing the Adobe Reader didn't convert anything. It simply told the OS to make itself the default app to use when you double click a PDF.

If you want Preview to be the default, right click on any PDF and choose Get Info. Under the Open With heading, choose Preview, then click the Change All button.

Edit: Too slow. Barney-15E already suggested the latter.

Technically, all PDF documents are Adobe PDF documents. They own the specification.

In Finder, select a PDF document.

Get Info

Select Preview from the open with popup menu.

Click Change All.

If you installed Adobe Reader with an Installer, you need to use their uninstaller to remove it. Otherwise, drag it to the Trash.