How do I maximize my fall distance?

Travel without parachute the most horizontal angle as possible
(you should drive at 126km/h when doing so).

Then when the parachute gets out,
tap Forward key repeatedly (~1-3 times/s) until you get close to the ground.

By the way, make sure you dont get over a hill top, or your parachute might open too soon.

Also if you open your parachute before, you will go further but in a non efficient way.

I'm using Hybris answer while playing but I've seen people going a lot further. Do go the furthest as possible from the plane's path, you open your chute as soon as possible but it will also take a lot more time for you to get to the ground.

From this reddit post :

you should be able to get close to two map squares if you open parachute early and rock back and fourth. i was watching a streamer and he was saying you want to keep speed at 47mk/h. i found that worked the best so far.

Also said here that this technique seems to be the best :

You want to jump out the plane and push yourself in the direction you want to go using your forward key "w" by default.

After you have pushed yourself in the direction as soon as you see your release parachute key pop up that it can be pressed, you want to press it at that point.

Once the parachute has been deployed you want to swing yourself back and forth just as if you you were on a swing set by holding the forward key and then letting it go as your feet come back.

Just repeat this until you reach your goal. You can also adjust when you release the parachute of course if the distance is not as far.