Crocodoc is a free, online service that allows you to upload and markup documents, including PDF files. You can not actually edit the PDF file, but can add text, highlight, draw, and even strikeout text. When you strikeout text, it opens a new text box above the strikeout for you to edit.
I used it recently to make a change to a physics lab I use with my students. It is a PDF file and I just wanted to make some small changes to the procedure. I uploaded the file, make the markups, and downloaded it. No account is necessary.
You can create an account and store files on the system. It even allows multiple people to markup and collaborate on work. Crocodoc works with PDF files, Word Documents, images, and PowerPoint presentations. You can view and mark up your files online. The documents can be shared with others who can all mark the files up collaboratively. You can make revisions, highlight or strikeout text, add notes and comments. Files are stored on their servers and can be password protected.
This is a great tool for educators and students to use for sharing work, working collaboratively, and providing feedback on work.