The PDF reading app that saves you time by sifting out the most important parts of your document for you using your highlighted text or notes.
Sift is integrated with Evernote so that you can use your notes that you already have there to find the most relevant parts of your pdfs as well.
You can collaborate with others within your documents inviting them to join your groups and make notes. You can also ask questions and discuss notes using comments.
Create your own topics and then cross reference your notes in the Summaries page. You can also visually group your notes in the Noteboard.