Safari is in Full Screen Mode.
When Safari is in Full Screen mode, Menubar and top bar are hidden.
Dock is also hidden.
To get out of the Full Screen mode,move the mouse pointer to the top of the screen and hold.
When the menubar drops down, click the
blue double arrow at the right most end of the menubar.
Pressing the esc key in the keyboard will also do the same thing.
Grey double arrow at the top right most part of the window is to make the window to enter Full Screen Mode.
You want the Safari window to fit the screen?
Once Safari is not in Full Screen mode,
move the mouse pointer to the bottom right corner of the Safari window.
Double arrows will appear. Drag it to fit the screen.
I have the same problem. Started when Adobe Reader asked if I wanted to have Reader become default PDF display type. Believe Preview uses Reader and did not check to have Adobe become primary.I understand what dominic23 says about resizing, but why did this happen in the first place? Just an electronic gotcha or an ongoing problem. BTW I trashed Adobe reader and can now view items in Preview and print.