how do I merge the tabs of two open Preview windows if I have more than two open?

This is doable in Safari, but I don't know how to do it for Preview.

Solution 1:

First of all, make sure that the "tab bar" is displayed by choosing "view"->"Show Tab Bar". Then, you should be seeing the tab bar, which displays the file name:

Then, in the other preview window, click and hold the tab bar, and drag it onto the tab bar of the target preview window. Using this method, you can merge as many tabs as you want.

Tested on MacOS Catalina 10.15.4

Solution 2:

Maybe I misunderstand your question, but here's what I think you're asking:

With two (or more) open windows in Preview (macOS 10.15.x & probably other recent versions):

In the menu bar, while one of the Preview windows is in focus:

1. click the menu bar item labeled Window
2. select the Merge All Windows item from the drop-down menu
3. all open Preview windows are merged into one window, and each (former) window is now a tab in the single Preview window

You can also drag tabs between windows in Finder. With two open Finder windows, and both windows set to View -> Show Tab bar, just drag a tab in one window into the tab bar area in the other window.

AFAICT, it works the same my browser windows do.

This may all seem a bit confusing. If so, here's a video that may explain it more clearly.