You can attach files (PDFs, MS Word or other documents) to items in your Mendeley Desktop Library by selecting the library item you want to attach to (in the Center Column), and then clicking on the "Details" button in the Right Sidebar Column. Scroll down to "Files" in the Right Sidebar Column, then select "Add file...". You can then add a file stored on your computer to the library item.
You can attach notes to items in the Desktop version of your Mendeley library. Select an item in your library, then click on the "Notes" button above the Right Sidebar Column. Start typing your note in the appropriate box.
With Mendeley, you also have the option to annotate the PDF files of items stored in the Desktop version of your Library. When you have a PDF file open within your Mendeley Desktop library, an annotation menu will appear above the open document. This menu allows you to highlight, annotate, select text and rotate the pages within your personal copy of the PDF document.
You can use or delete tags and keywords already associated with items in your library, and/or you can create your own unique tags to keep track of Mendeley Library items. Tags may already exist, especially if a document was added from one of your groups. Keywords, often pre-assigned to the document by the author or an indexer, are listed, and you can add to the existing list. Be sure to include semi-colons between tags and keywords you add to the list. Once you have added "Tags" and "Keywords", you can use them to filter your documents, and further refine your collections.
Click on the PDF icon to the left of your item entry. Click on the "Edit tags" drop down arrow. Type your tags into the text box and click save. If you want to edit a document's keywords, click on the drop down arrow for "Edit document details" to the right of "Edit tags". Scroll down to find the "Keywords" text box, then type your tags into the text box and click save. Be sure to include semi-colons between tags and keywords you add to the list.
Select an item in the Middle Column of your library, then click on the "Details" Button in the right hand column. Under the bibliographic details and abstract are text boxes to add you own tags and keywords for the document. Tags and keywords can be added by typing directly into the appropriate text box. Be sure to include semi-colons between tags and keywords you add to the list.