Q: How do you find preferences in safari?

There is no preference to select one browser over another as the default browser in iOS. Safari IS the default browser on the iPad and you cannot change that, unless your iPad is jailbroken. Is your iPad jailbroken?

That's probably what you did. You can select any browser that you have on your iPad in the Open In window in the mail app, as long as the file can be opened in a browser.