When reviewing PDFs, you’ll notice a Project Manager icon on the right side of your screen:
Clicking the Project Manager icon will open a pop-up that lets you add the document to a new Project. If you have existing Projects, you can also add the document to one of them.
This icon is also accessible from your Documents History tab, allowing you to add documents to Projects directly from your list of recently viewed items. You can add as many documents as needed to each Project. (We're also working on enabling tagging of links in Projects in addition to PDFs—stay tuned for this feature.)