Yes, this is possible. Please see the images below
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 (see image 'Endnote_Find_PDF').
To highlight, click the 'Highlight Text' button in the ribbon at the tom of the EndNote screen. Then, you just have to select the text that needs to be highlighted (see image 'Endnote_highlight_PDF').
To annotate, click on the 'Sticky Note' button and then click on the part of the text where you want the annotation to appear (see image 'Endnote_Annotate_PDF').
You can see that there is a 'Find...' option, at the top of the screen, which allows you to search in the PDF text as well.