At the bottom of the left area you see a [+] button on the left, which offers the same functions that you get via the [Import PDF] button and the [New Group] button in the toolbar above.
Right to it, there is a [–] button which is only selectable when at least one PDF or group has been selected in the Collection list above. Use it to drop the selected PDFs to the trash or to remove groups (and push their content to the Trash). This is equivalent to clicking the [Remove] button in the toolbar above.
At the very right of this accessories bar, you see three vertical lines. They symbolize a mover which you can grab and move to change the width of the left area of the document window. You can also place the mouse pointer exactly on the vertical separation line between the left and middle area of the document window, until the mouse pointer changes into a mover with arrows to the left and right. Then you can also change the column width.
