I'd also suggest scanning the downloaded file on virustotal.com before sending it anywhere :)
If you need to find any books, check out Annas Archive at https://annas-archive.org/ . Download any book you want, then you can just email it to your Kindle.
Discussion
Thanks
Getting an error message when sending an epub to my send to kindle address. Error message doesnt tell me anything, just that it fails (repeatedly). Any idea what that could be? Sending pdf was no issue.