Dropbox said Monday that it's acquiring electronic signature start-up HelloSign for $230 million in cash, its largest purchase ever.

The move puts Dropbox in competition with Adobe and DocuSign, and adds functionality that can potentially lure more big businesses to the company's core file sharing and collaboration products, where it contends with Apple, Google and Microsoft.

Although Dropbox has exceeded analysts' estimates in each of its first three quarterly earnings reports as a public company, the stock hasn't done much to reward investors since its debut in March at $21. The shares fell 0.3 percent today to $23.84.

"Coming out of the year of the IPO, the next question, of course, is, 'How are we going to continue to broaden the job we do for customers?'" Quentin Clark, senior vice president of engineering, product and design, told CNBC on Friday.

HelloSign users can apply their signatures to digital documents and ask for signatures from others. Companies can embed the HelloSign signature system into their own software, and the technology is available inside products from Google and Salesforce, as well as Dropbox.

HelloSign also has tools for virtually faxing documents and managing the flow of paperwork, but the signature tool is the most popular, said Whitney Bouck, HelloSign's chief operating officer and a former Box executive, in an interview.

HelloSign will continue to operate independently, for now at least, with CEO Joseph Walla reporting to Clark. While Dropbox's sales team will promote HelloSign to customers, it will continue to work with other electronic signature providers.

"We have to maintain this neutrality," Clark said.

HelloSign was founded in 2010 and, like Dropbox, is based in San Francisco. Investors include Alphabet's GV, Greylock Partners, U.S. Venture partners and Y Combinator. HelloSign has around 80,000 customers, including Intuit, Lyft, Samsung and Twitter.

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