Although public cloud storage systems like Dropbox, Google Drive or OneDrive, are free and easy to use, you should avoid them for sharing sensitive files. You have no control what happens with your documents as you are dependent on the provider to keep your information secure.
An alternative cloud-based solution which comes with automatic encryption is ‘BlauCloud’. The service is based on the open-source cloud solution ‘ownCloud’. As the data is stored in Germany, users benefit from more restrictive German data security law. Even then, this method requires you to trust the host with your data.