can't use word and preview on the same screen

So lets say i have 1 word window open at a full screen and on top of that, i have preview on the left half and word on the right half. So I am currently seeing word on the right and preview on the left because i need to copy some information down from a pdf. I don't know why, but when I click on word, both word windows come up and the pdf preview windows goes behind the full screen word window.
How can I have preview window and word window at the same time while having a full screen word open in the back?

 

sorry, full screen as in "larger window" not actually full screen that you have to swipe the touchpad to get to...