That was more difficult than I anticipated: [Wayback/Archive] how do I merge the tabs of two open Preview windows if I have more than two open? – Ask Different
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