Question: Where is preview tabs in sierra?!
Hi. I had this question but since the forum was old I couldn't reply after I found the answer.
follow the steps to enable tabs in preview (if you cant find see them by default)
View --> Show tabs
and then open another PDF or Image or ... and then hold the tab of that separate window and drag it the the first open PDF or image or ....
MacBook Pro with Retina display, macOS High Sierra (10.13.3)