First you need to add a custom group to the ribbon tab you prefer, probably the Home tab. Then you can add the button to that group (you can't make any changes in the built-in groups.)
1. Click File > Options > Customize Ribbon.
2. In the list on the right side, click on the name of the tab where you want the button to live.
3. Below the right-hand list, click the New Group button. Use the upward arrow to the right of the list to move the new group to the position you want. Click the Rename button and enter the name you want the group to show on the ribbon. Leave the new group selected in the list.
4. On the left side, set the "Choose commands from" dropdown to "File Tab". Then scroll the list of commands until you can click on "Publish as PDF or XPS".
5. Click the Add button between the lists. The command will appear in the new group.
6. Click OK. The group and button will appear on the selected tab of the ribbon.
You can do the same to the Quick Access Toolbar, where you won't need any group.
The button opens the Publish as PDF or XPS dialog, which looks a lot like the Save As dialog, with PDF already selected in the Save As Type dropdown. If you want XPS instead, you have to change that dropdown.