We were tasked to build an online PDF editor. The app functionality had to include such features as uploading files to the system and viewing them in a browser, sharing a file with other users, adding text and graphic comments. We were also excited to collaborate with another team, because it is always a useful experience.
We ended up creating an excellent solution for multi-user PDF editing. Once you've logged in, you can upload a file and share it with all of your team members. Every one of them will be able to view the document in a browser, use a handy toolbar, and add text comments and graphic elements: scalable lines, circles and rectangles. All applied changes are saved on the server and visible to anyone, who has access to the file.