Go this this page on your Kobo/Kindle ereader and you see a unique key. Enter it in this form and upload one or more ebooks and they will appear as download links on the ereader.
If you send an EPUB file to to a Kindle it will be converted to MOBI with KindleGen. If you send a MOBI file to a Kindle it will be sent unprocessed. Files sent to Kindle eReaders will have their names stripped of special characters, a limitation of the Kindle browser. If you send an EPUB file and tick the Kepubify checkbox, it will be converted into a Kobo EPUB using Kepubify. If you send a MOBI file to a Kobo, it will not be converted.
Your ebooks will be stored on the server as long as your Kobo/Kindle is viewing the unique key and is connected to wifi. They will be deleted irrevocably when the key expires about 30 seconds after you close the browser, generate a new key or disable wifi on your ereader.
By using this tool you agree that the ebooks you upload are processed on the server and stored for a short time.