Hello! I'm trying to download FDM on Manjaro Linux (Arch Based) and I found the program on Flathub, but I didn't find any mention about it here on the FDM website, does anyone know if this version is official or reliable?

https://flathub.org/apps/org.freedownloadmanager.Manager

It's not an official build. It's clearly described on the linked webpage:

github wrote:NOTE: This wrapper is not verified by, affiliated with, or supported by FreeDownloadManager Team.

What is more, you can easy search for an answer in older topics:
search.php?keywords=flathub&terms=all&author=&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=-1&t=0&submit=Search

FDM is available on Arch's AUR repo. You could try that, though it is community maintained.