Mendeley is freely available as:
Your Mendeley library can be synchronised across platforms, so you can access your Mendeley library from any device.
With the free version you have 2GB of personal storage space. It is possible to pay for a premium plan which gives you more storage space.
A legacy version, Mendeley Desktop, is no longer being developed and no longer available to download.
Please note that Mendeley is not accessible to screen readers. If you use a screen reader you might prefer to use EndNote or Zotero instead.
A free Mendeley account gives you
To see how much storage you have left: