We have integrations with a variety of tools, including popular cloud services like Box, fileee or Evernote. You can find an overview of the possible integrations here. We're constantly working to increase our integration offerings. Let us know which integration you would like to see next.
Alternatively, you have the option to download your letters and store them in a cloud service of your choice.
Mobile also allows you to upload your documents to a cloud service of your choice without downloading.