Why is the "download" link dowloading a .txt file

[Why]

Which then opens up a webpage asking to sign up. No program file downloaded. Very dodgy.

[Zenju]

You mean the link on Sourceforge? It's a hack as Sourceforge doesn't offer a way to directly redirect to freefilesync.org.

[xCSxXenon]

Always use the source website!
https://freefilesync.org/download.php