If you have a subscription to ReadCube, your content will be syncing through ReadCube's Cloud service. You can check your online library to verify that your content has synced successfully.
However, having a local backup on your computer is always a good idea - especially if you're preparing to reinstall ReadCube or migrate your content over to another program.
To do this, launch the ReadCube desktop application and head into Preferences and Support. Once there, you'll see the option to export your ReadCube content as a ReadCube Archive (.rc) file:
If your PDFs are not local on your computer, please be sure to check the "export files" box as well or only the metadata of your ReadCube library will be saved.
Once you click on "export to ReadCube Archive" a .rc file will be generated. Should you need to restore your library, you can drag that .rc file directly into your ReadCube library and all of your content will be restored.