Crocodoc is a file sharing website where you can annotate and edit PDF files, Word documents, images, etc. Using Crocodoc allows students to annotate and collaborate while engaging in literacy and writing.
TLC 2013 presentation on using Crocodoc Personal and PaperPort Notes to engage students in critical reading and close reading strategies.
This video provides background on close reading and expectations involved with the Common Core State Standards.
Teachers may want to consider how close reading will improve student reading ability and how they can implement technology to increase literacy skills.
This blog post provides a top ten list for annotation apps for iPad. I use PaperPort Notes in my classroom.