Double-click on the citation containing the PDF in question. If you already have a PDF attached to your citation, you should be able to access it by clicking its name at the top of the screen next to the 'Reference' tab:

Searching the PDF

To search the text of your PDF, use the 'Find..' button located at the right hand side of the toolbar.

Highlighting

To enable the highlight tool, click the 'Highlight Text' button in the ribbon at the top of the EndNote screen.
You can use the tool to select the text that needs to be highlighted.

Annotating

To enable the annotation tool, click on the 'Sticky Note' button and then click on the area of the text where you want the annotation to appear.

Lots of further EndNote tips and guidance can be found on our 'EndNote Support Libguide page'.