The most reliably complete citations for journal articles are found in the database holding that article. You are usually more successful if you start from a page like this, rather than from an open PDF:

To add a citation like this to your Mendeley account, click on the 'Add to Mendeley' button in your browser

If you have not logged into Mendeley lately, you will be prompted to do that.

To make sure all the citation details are correct, click on the pen icon to edit the citation information. To save the citation, click on 'Add' (though you can also specify the folder to which you would like to add this citation. (You would have needed to create this folder beforehand.)
