We are pleased to announce that we worked with Adobe to improve access to PDF files. You can now open PDF files directly in Adobe Reader – and the PDF file will remain connected to SharePoint Online. You can also edit and save your changes to SharePoint Online from the desktop. And further, you can now use versioning (check-in and check-out) with PDF files for better management and review. We have improved the experience while maintaining the level of security you expect from Office 365.
The SharePoint Online Team