A quick tip on selections in Library Mode.  Selections allow you to select several items at once so you can add metadata – keywords, ratings, etc. with one click for all instead of each one individually.  Here are a few shortcuts to remember.

CTRL/CMD A:  Selects all the items in a folder or grid.

CTRL/CMD Click: Holding the CTRL/CMD key down while clicking on items selects individual items not together.

Click > Shift Click: Click on the first item in the grid you want to add a keyword to and then click on  the last item (i.e. all the papers) while holding the SHIFT key on your keyboard.

A few more misc. shortcuts you may find helpful while working in the grid:

CTRL/CMD+D: Deselect all items in the grid (this is really helpful after selecting CTRL/CMD + A).

/ : In a group of selected items if you have one item you do not want selected, you can click on it and then click the / key to deselect it.  You could alternatively just CTRL/CMD Click to deselect an item as well.

Hope these few small shortcuts make your work in the grid a lot easier as you work on adding metadata.