There might not be anything wrong with the file you downloaded. You might be trying to open an .epub file without an .epub reader program.
What is an .epub file?
An .epub file is an electronic book file that uses the file extension ".epub". The term "epub" is short for "electronic publication"
How to open an epub file
You can send it to your Kindle app on your phone or tablet using this URL https://www.amazon.com/sendtokindle
You can open epub files using free apps like Apple Books, Adobe Digital Editions, Thorium Reader, Dolphin Easy Reader, and EPUB File Reader
There are some free website that can open an epub file like https://epub-reader.online
You can also open epub files on desktop computers and mobile devices Calibre (free tool) https://calibre-ebook.com
How to convert an epub file
You can convert an epub file to a PDF using Adobe Acrobat
You can convert an epub file to a Kindle-compatible format (MOBI or AZW3) using a conversion tool like Calibre or an online converter
How do I know it's an epub file
For Windows you might need to turn on your file extension in your Windows Explorer. Click on the "View" tab and make sure "File name Extensions is checked mark is checked. Then you'll see the .epub file vs a .pdf file
