The Quick Access Toolbar

At the top left corner of the application window, you can find the Quick Access bar:

By default, this toolbar contains shortcuts to the Open File, Undo and Redo commands. However, it is fully customizable and you can add whichever commands you want or remove the existing ones.
Right clicking anywhere on either the Quick Access toolbar or the ribbon will open this context menu:

Customizing the Quick Access toolbar

This customization dialog allows you to fully customize the Quick Access toolbar.
Using the dropdown menu at the top, you can open lists of commands from the ribbon, or simply a list of all available commands, then just press Add to add the ones you want to your toolbar.
Once you've added the commands you want, you can freely change their order inside the toolbar using the up and down buttons on the right side of the dialog, to make the toolbar look exactly the way you want it to. Of course, if you change your mind you can also pick any command you've added and press Remove to remove it from the toolbar.
Pressing the Reset button will reset the toolbar back to its default state, with the same buttons it had when you first installed PDF2XL.

Moving the Quick Access toolbar

Selecting Show Quick Access Toolbar Below the Ribbon from the context menu will move the toolbar from the top of the window to just under the ribbon. You can always move it back by right clicking the toolbar again, and selecting Show Quick Access Toolbar Above the Ribbon.

Minimizing the Ribbon

Selecting the Minimize the Ribbon option will hide the ribbon's contents, leaving only the tab names visible. Pressing on one of the tabs will show that tab, allowing you to use its contents as usual. Afterwards, or if you click elsewhere in the window, the ribbon will disappear again.