For hosting common files it's ok. Also it is on Android too do, great to transfer files between your Windows and your phone. But if you want better security you have to subscribe a monthly offer.
Universal Book Reader - Just like its name, Universal Book Reader is really a universal book reading application allowing you to read the eBooks from the convenience of your smartphones.
Dropbox - Online Sync and File Sharing
Cool Reader - Fast and small cross-platform eBook reader for desktops and handheld devices
Google Drive - Access and sync your files anywhere
iReader - iReader is one of the most accomplished ways to read the books right from your smartphones.
Mega - Secure File Storage and collaboration