Apart from media, all kinds of documents can be uploaded to a Craft document. Editable documents such as Word, PDF or Excel files can be opened from Craft if you're using it on a Mac, iPhone or iPad. Craft for web will open those in a separate tab in your browser.
Note: Craft will not save any changes done to a document you open from the app. To go around that limitation, please make sure to save the edited document on your device after editing it.
It is not possible to open a document file from the Craft for windows app.