To access your files from here, you need to download them. If you're doing this from a phone, you may need to save them into your "downloads" or "files" folder.

If you use the Calibre book management software, I'm guessing you have this under control. :)

KINDLE:

If you use a Kindle, you can email the .epub version of the file to your device. Access your Amazon account (Content Library/Devices) to look up the email address to your device.

NOTE: Make sure you've whitelisted the email address you're sending FROM.

To whitelist the address you're sending from, go into Content Library/Preferences, and scroll down the page to Personal Document Settings, then scroll down there to Approved Personal Document E-Mail List. At the bottom of that section should be a link Add New E-Mail Address.

You can also use this webpage from Amazon to transfer files: https://www.amazon.com/sendtokindle

APPLE DEVICES:

Download the file to your device, then select either the Open in Books link that appears, or use the menu to "Send to Books". Depending on your default browser, if it opens as a separate webpage instead, look for the "send to" link on the browser and that should give you the option to open in Books. (Note: It might be on a second page depending on your your phone is set up.)

NOOK:

Read about it on this link: https://help.barnesandnoble.com/hc/en-us/articles/5601646565403-NOOK-Tablet-Devices-Sideloading-Files