You can easily add documents saved on your computer to your Mendeley Desktop Library. Mendeley can often capture bibliographic information from these saved documents. You can also save documents to your web library, but that may require entering in all the bibliographic information yourself.
There are two ways to add a saved document to your desktop library. The first is to click on the icon with a green plus sign. This will open a dialog box directing you to find a document saved on your computer.
The second method is to drag and drop the document into the Center Panel when the appropriate collection foler is selected in the Left Sidebar Panel.
Click on the "Add Document" button. This will cause a pop-up box to appear in which you can select which folder to save the item to, and select the type of document you are saving. An additional box will appear for you to record bibliographic information for the document you are saving.