Sort
You can sort the documents in your library, for this there are several options
- sort by name
- sort by date
- custom
In the case of a "Custom" sort, once 'Custom' has been selected in the 'Sort by' drop-down list, click on the icon 
immediately to the right, it must be highlighted in blue.
You can select the documents by clicking on their thumbnail, and drag and drop them in the desired order.
Once the order has been defined, click again on the icon 
which is highlighted in blue, to save the custom order, it must go back to white.