Crocodoc, a startup incubated by Y Combinator, recently launched an easy way to annotate documents on the web. Today, it’s speeding that process up, by displaying collaborators’ annotations in real-time.

Existing annotation options tend to be expensive, complicated, or limited to a specific format. Adobe Acrobat Pro, for example, costs $449 and is limited to PDFs (although it includes more than annotation). Microsoft Word includes some annotations features, but again, that’s just for Word documents. With Crocodoc, on the other hand, you can upload a Word, PDF, or PowerPoint file, or a JPEG or PNG image, then add annotations like text, highlights, strikeouts, and Sticky Notes.

Each document gets a unique, randomly-generated URL that you can share with collaborators, so they can view and annotate a document without a Crocodoc account. When you’re done annotating, you can export it as a PDF.

Until now, however, if you wanted to see remarks left by people annotating documents at the same time as you, you had to refresh the page. Since you aren’t editing the document itself, it’s not a disaster if you and a collaborator make annotations that overlap with or contradict each other. Still, it sounds kind of annoying. Plus, chief executive and co-founder Ryan Damico said users had just come to expect that kind of real-time responsiveness from other web services. The company plans to improve this feature in the coming months, for example by color-coding each user’s annotations so you know who did what.

Crocodoc is free, although it also offers a pro version that includes security features like password protecting your documents. It just integrated with the Google Apps Marketplace, so Apps users can import their annotated documents into their Google library.

The company is demonstrating at today’s Y Combinator Demo Day, where it’s announcing the real-time collaboration features.