How to fix what I did?

I know what I did, I just dont know how to technically describe it to ya'll.

Every finder window has a back and forward button on the top left toolbar, its right under the 3 colored buttons. I accidentally drag and dropped a pdf file to the left of the "back button". The whole back/forward button thing, moved itself over to the right. Now there is an empty space to the left of it, (nothing shows, like a button or icon or anything, just empty space). If I click that empty space, the pdf opens. I created some kind of shortcut, is this a feature in mac osx? If so wtf is it called? And how is it useful?

Lastly, how do I get rid of it??

No matter what finder window I'm in, if I click that empty spot, the pdf file opens up.

 

As soon as I right click, the icon appeared. Gone now. Thanks!