**Executive summary:**
It seems like the downloaded file is compressed and you should decompress it.
**Detailed answer**
Running:
wget
Result with a downloaded `index.html` file
Executing `file` command on the download file shows:
$ file index.html
index.html: gzip compressed data, from Unix
Renaming the file and decompressing it turn it to be HTML document
$ mv index.html index.html.gz
$ gunzip index.html.gz
$ file index.html
> index.html: HTML document, UTF-8 Unicode text, with very long lines, with overstriking
**Extra Info - why wget downloaded a compressed file?**
As explained in How To Optimize Your Site With GZIP Compression:
Instead of downloading a large text file, modern HTTP server/clients uses _Compressed HTTP Response_ which reduce the size of the transfered files.