You can check the link and you will be able to verify it is coming from the official source. Try downloading Avast offline installer from Softpedia and check the link.I use SP to follow the update release of apps, but when I want to dowload the new installer, I move to the app official website.
I know SP is trustworthy, but I prefer to stay more secure and avoid any 3rd party website for installers.
Btw there is a fantastic hack when using SP. Create an account there and add the programmes you are using to your wishlist. Now whenever there is any updates for the programmes you added to the wishlist, you receive a notification email from SP.
The email looks like this



