Cloud services like iCloud, Dropbox, and Google Drive help you store files online, so you can access them from any computer or device. If you're unfamiliar with them, ask a friend who has experience before getting started with one.
Know Me Toolbox can get PDFs from most cloud services to put on your Bookshelf. To make sure Know Me can access your cloud account(s):
Install the service's app on your iPhone or iPad (for iCloud, this means the "Files" app)
Open that app and log into your account