If you check this option, we'll convert your MS Office files be it DOCX, XLSX, PPTX or any old format into our own Zoho Office editor formats. Once converted, if you open such files, they'll always open in our Zoho Office editors. This will allow you to share and collaborate with your colleagues and friends in real-time. 

If you choose to uncheck this option, this basically means you want to retain the original format that you upload. In that case, we'll only allow you to preview such files using our new preview UI. You will not be able to edit them using our Office editors. You can still share the files with your colleagues in Read/Write mode. Your colleagues can upload a new version of the same file after editing it in MS Office. 

Previously, if you uncheck this option, we opened and saved back as MS Office files itself. So why did we change it ?

If you wish to edit MS Office files using our Zoho Office editors, you can convert them to our editor format and continue to edit. But in doing so, we'll only create a copy and not touch your originally uploaded MS Office file. This has several advantages. Zoho Docs being a file storage service, it shouldn't change/alter the file that you upload to Docs. In keeping with this principle, you can now confidently upload any file to Docs. We'll have the original format intact for the lifetime of the file. 

For now, only the authors who own or created the file can do the conversion to our editor formats. Moreover, you still have to re-share the file to your colleagues. Because the new file will not retain the sharing permissions of the originally uploaded files. We'll see how we can improve this behavior.