Crocodoc, a startup that offers a free tool for annotating documents, just announced that it has raised angel funding from high-profile backers.

The investors include Dave McClure, Delicious founder Joshua Schachter, Gmail creator and FriendFeed cofounder Paul Buchheit, YouTube cofounder Steve Chen, and XG Ventures. The size of the round was not disclosed.

Crocodoc was part of the recent class of startups incubated at Y Combinator. Its goal is to present an easy and affordable alternative to programs like Adobe Acrobat Pro, one that allows you to annotate multiple document and image formats including Word, PDF, PowerPoint, JPEG, and PNG.

Cofounder and chief executive Ryan Damico said the team has spent the last three months revamping the features and interface, and then rolling out those improvements over the last week. The most notable additions are richer commenting tools. For example, users can now draw an area around the relevant portion of text, then leave a comment, then start a conversation in response to that comment. This seems so much more powerful than the previous Sticky Notes feature, Damico said, that the Sticky Notes have been removed, at least for now.

Crocodoc has been used primarily by individuals, but the company plans to “make a concerted effort” to convince organizations to give it a try as well, he said.

“This is a huge step forward towards really becoming the best universal document viewer and collaboration tool,” Damico said. “That’s the niche that we’re going to carve out.”

Crocodoc’s basic tools are free, but it charges for a pro version. The company is based in Silicon Valley and Boston.