1. Tap one of the file browsing tabs (Recent, Folders, All Documents or SD Card).
2. You can then access the Favorite button using two different methods:
Method 1: Long press the file to bring up the file management options. Tap the Overflow icon on the right hand side of the top toolbar to show Add to Favorites.
Method 2: Tap the file's Information button, to show the Favorite icon.
If you're accessing the file from the Recent tab, you can long press on the file, to show Favorite icon. You can add Google Drive and Dropbox files to Favorites from the Recent tab, after they have been opened once.
Now your files will appear under the Favorites tab.
Creating a new PDF (blank or from an image)
Creating a folder
Saving a copy of a file with a new name
Deleting a file