Shortly after its 2006 launch, Scribd.com stumbled across an easy way to explain what it did: it called itself "YouTube for documents". The phrase sounded snappy enough, and even if was a little confusing, at least it had the virtue of being easy to use in elevator pitches.

These days, however, Trip Adler - the company's fresh-faced chief executive - seems a little ashamed by the idea.

"In a startup, in the early days, it can be hard to explain what you do," he says, trying to excuse youthful transgressions. "It worked in the first few weeks. We got a lot of buzz ... people didn't know our name, but they knew that's what we were."

Since YouTube is one of the world's most popular sites, such a bashful attitude could seem misplaced. But the reason for Adler's embarrassment is straightforward: three years after those early fumblings, Scribd is getting enough traction to stand on its own two feet.

Documenting success

The site now ranks as one of the world's top 100 websites and, with more than 35bn words in its system, it is increasingly the place to go for documents of all sorts. It contains everything from academic papers and PowerPoint presentations to recipes and electronic books: all searchable and shareable for the site's estimated user base of 50 million. Adler may not like the analogies, but there is a truth in the comparison: what YouTube is for video, or Flickr is for photography, Scribd is fast becoming for the written word.

Not bad for something that started out as an offshoot of an entirely different project. The site came about after Adler and his college friend Jared Friedman (now Scribd's chief technologist) applied to join the Y Combinator programme in 2006 - a scheme that offers a small amount of money and extensive mentoring to young entrepreneurs in exchange for equity.

After a summer spent plugging away on unsuccessful ideas (including a transportation website and a classified advertising site for students) the duo joined forces with a third inductee, Tikhon Bernstam, and came up with a new concept.

"I was talking to my dad, who's a neurosurgeon," says Adler. "He had this academic paper he wanted to publish. Journals take about 18 months to publish a paper and he just wanted to get things up there."

"That inspired the idea of a website that would let him share the files easily ... but we realised it was much bigger than that, and could apply not just to academic papers, but to books people write, short stories, PowerPoint presentations, recipes, sheet music and anything people want to publish online."

The site operates today by opening that concept up to the world. Users can scan in documents of any sort, before they are converted into a Flash-based reading format (they call it iPaper) or made available to download or share online.

The company itself looks like a standard startup - it has just shifted to a large, loft-style office in the middle of San Francisco where the staff can blow off steam by riding scooters and go-karts between their desks. There's one startup cliche that Scribd is avoiding, however. "We're probably one of the few startups in Silicon Valley that's profitable at this stage," he chuckles.

Indeed, he says the company is making enough cash to cover its technology costs and the salaries of 28 employees - unusual for a venture-backed startup, let alone one that raised $9m of financing last December. While the team's plan for aggressive expansion may eat into that - and there are a lot of empty desks waiting to be filled - it's still a surprise to hear mention of profit.

Adler thinks the reason why the site makes money is straightforward: it's got something people want. "The reason we make money is because we have a few different business models," he says. "One is ads: we get incredibly high click rates because most people on Scribd are searching the site for something, or they came from a search engine and they're looking for something specific."

"Then," he adds with understatement, "we also have the Scribd store." The store has become the site's hottest feature, and the one that is getting attention with mainstream publishers, authors and the public. It's a simple idea: that anyone can charge a fee to get access to their documents online, giving a percentage to Scribd for hosting the service. It offers a range of options for content owners to choose what they are selling - and it's delivering results.

Going social

Last month, the site signed a deal to sell 5,000 ebooks from publisher Simon & Schuster - bringing bestselling writers including Stephen King and Dan Brown to the platform. Other recent converts include Harvard University Press, while for a limited time Wired's editor-in-chief Chris Anderson is giving away his new book, Free, on the site.

With such interest, the move has proved instantly profitable. Scribd takes 20% of the cover price, and the remaining 80% goes to the publisher. That split remains the same across the board: even Simon & Schuster did not get a sweetheart deal to tempt them in, says Adler.

But despite courting the notoriously stuffy publishing industry, the site hasn't entirely dispensed with its web 2.0 roots. Adler is keen to stress that professional content is just one aspect of what the site does - and that allowing people to share documents they have created themselves "democratises" publishing. The site also has plans to launch what it calls Project Social in the near future, which will let users follow their favourite authors and publishers to see what they put online.

"The goal is to become the central hub of publishing, where we have all the written material - user-generated and professional," he says. "We want to be the place where people can publish instantly to their audiences ... and to get there, it's just about doing things step by step."

By the book

5,000 Number of ebooks Simon & Schuster deal will bring to the Scribd.com site
35bn Words in the Scribd.com system - users can search for shareable documents
20% Share of the cover price taken from documents sold on the site