If you have a PDF file on your computer you can drag and drop it over the Mendeley Desktop program. Mendeley will extract the data and add your reference automatically.
Using the File menu in Mendeley Desktop, you can choose to add individual files or the contents of an entire folder by browsing to the relevant location on your computer.
You can also opt to ‘watch’ a folder - which will mean that Mendeley monitors that location for any new items being dropped into the folder. If it finds a new document, that document will be automatically added to your Mendeley Library.
In Mendeley Web you can add files using the '+ Add' dropdown menu. Choose to add a document or folder, or import a library in .xml, .ris, or .bib format.
If the PDF was not automatically added to the citation information when importing, you can add it later, using Mendeley Desktop.
Click on the record you would like to add the file to, and scroll down the right-hand panel to 'Add File...' Now you can choose the PDF from where you have saved it and attach it to the record.