the best are
is a fully online PDF editor, which will let you do notes, comments, highlights and drawing on a PDF, which you can then download or share with anyone, and you donít even have to log in. The only thing you canít do is actually edit the text of a PDF file, so this is more for annotating an existing document (e.g. class notes) than re-editing.
is very similar to PDFZen, with similar capabilities and similar tools, but a slightly different interface.