If you upload a PDF, you can highlight and annotate specific passages in the PDF view. Look for the highlight text and comments options in the top menu.
If in a shared folder, annotations are tracked with commenters' names to the right of selected passage.
You can make notes about the entire document in two places.
In the PDF view, look for the document icon in the upper right. Notes in here will be searchable across all your citations and documents, but you will not be able to specify it as a field in the Advanced Search.
In the edit a citation view, there is a field for Notes that you can use to track notes about the document. This field is searchable, and you can specify to search only that field in the Advanced Search. This method is useful if you are not uploading PDFs to your citations.
