The sensitive information was found via a publicly available search engine that is part of Office 365.
Security researchers said many firms mistakenly thought documents would only be shared with colleagues not globally.
Microsoft said it would "take steps" to change the service and remove the sensitive data.
Security researcher Kevin Beaumont discovered the sensitive information after using the search engine on Docs.com - a website that is part of the Office 365 online software service.
Many firms use Microsoft's well-known suite of office productivity programs by subscribing to Office 365 which also gives them access to online services including Skype as well as a document-sharing and storage system.
In a series of tweets, Mr Beaumont revealed some of the sensitive information he had found via the Docs.com search engine.
"People clearly don't understand how the service works. It defaults to publicly accessible, which is the problem," he wrote.
Other security researchers followed up his discovery and unearthed confidential business papers including lists of passwords and access codes as well as social security and National Insurance numbers.
Many users complained to Microsoft via social media about documents being exposed publicly. The software giant initially reacted by removing the search box from the main Docs.com page.
However, security experts following developments found that this did not remove all the exposed documents from view.
"Files were still cached in Google's search results, as well as Microsoft's own search engine, Bing," wrote Zack Whittaker from tech news site ZDNet.
Microsoft later took steps to block incoming searches from Google to stop information being found.
However, on 27 March, the search box returned to the homepage of Docs.com.
In a statement shared with several news organisations, Microsoft said: "As part of our commitment to protect customers, we're taking steps to help those who may have inadvertently published documents with sensitive information."
It added: "Customers can review and update their settings by logging into their account at www.docs.com."
Confidential documents, passwords and health data have been inadvertently shared by firms using Microsoft's Office 365 service, say researchers.