Can I attach documents from Dropbox, iCloud, Google Drive and others?

Yes! On iOS devices, tap on the attachment icon and then tap on Choose Files.

Then under Browse, tap on one of the options in the list and select the files or documents you want to attach. 

If you do not see the service you are looking for, you will need to enable the app. 

Tap on the ... in the upper right and then tap on Edit. 

Toggle ON the apps you want to use and then tap on Done.