Google Drive is pretty great, as long as your files are in Google's format. You can certainly open other types of documents, like Microsoft Office files or PDFs, but you can't do much more than look at them (or convert them to Google Docs format). If you regularly work with non-Docs files, you'll be happy to hear that you can now comment on more types of documents.
Drive users can now add comments to Word, PowerPoint, Excel, PDF, and image files. This improvement, like many recent changes to Drive, is primarily designed to help G Suite users. Many companies and organizations have a massive collection of files in older file formats, and converting them to Docs files often causes formatting to be lost.
- Google Blog