ProfHacker - The Chronicle of Higher Education
Diigo offers the only means to highlight passages and add annotations to websites from within the iPad's native Safari browser.
This is a post several of you have been looking for, I know this from the emails I have been receiving from you. Now you have a great video tutorial on how to use Google Drive app to create documentation, spreadsheets, tutorials, and PDF annotation all from the palm of your hand using your own iPad.
The iPad app GoodReader allows you to annotate on top of PDF files, this page will give some tips and tricks for editing the content of PDFs on desktop systems.