PDFVue is a new tool for both viewing and annotating PDF files. Similar to services like PDFMeNot and PDFHammer, PDFVue can open up PDF files right in your browser, forgoing the need to use desktop software. Better yet, if you've got the service's new browser extension installed it can be set to automatically open up any PDF link you come across while browsing. This is not the fastest process since it first has to download and render the file, but if you've ever experienced hang-ups with Adobe's Acrobat Reader it's a nice step up.
So why use this service other than for speed? One good reason is the annotation tools. It includes all the usual goodies that let you fill in forms and highlight text. It also lets you upload images to stick into the PDF, making it a lightweight desktop publishing tool, since when you're done you can save it as a separate PDF file.
One thing users may not enjoy about PDFVue is that it doesn't take advantage of your screen real estate very well. Documents are kept to the very middle of the page and cannot be expanded to take up the whole screen. If you're on a desktop computer with a nice big screen this isn't a problem, but when using it on a laptop I found myself having to zoom into a level that required quite a bit of scrolling to navigate around the document. This can be somewhat remedied by minimizing the tools window on the right side of the page.
Related: Preview PDFs in your browser without downloading them
PDFVue's PDF editor and viewer is full of tools to that let you mark up PDFs.