Go this this page on your Kobo/Kindle ereader and you see a unique key. Enter it in this form and upload an ebook and it will appear as a download link 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. 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 ebook will be stored on the server as long as your Kobo/Kindle is viewing the unique key and is connected to wifi. It 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 ebook you upload is processed on the server and stored for a short time.