The .pak files for Homefront The Revolution are encrypted, and thus cannot be opened by renaming the file extension to .zip (This is how you'd be able to extract .pak files in most cases)

Screenshot of Example using 7zip below.

hf2decrypt is a tool by XeNTaX that is able to convert the encrypted .pak files into a extractable
.zip file.

Firstly in order to download this tool you need to go to the website below and download the file, (only one I could find online) though I'm sure if you looked it up on archive.org you could find it also ;)

https://disk.yandex.ru/d/0Irz7uCA3YmV4b

Now that you have downloaded hf2decrypt you need to go to the game directory and extract the .zip file there.

C:\Program Files (x86)\Steam\steamapps\common\Homefront_The_Revolution\gamehf2
(This is where the game directory is normally located)

Now that you've done this you need to drag the desired .pak file into the hf2decrypt.exe and it'll convert it to a extractable .zip file

Now all you need to do is use your prefered file extractor to unpack the file .zip file
(I'd recommend 7zip.)

If you plan on modding the game you might want to check this out it might come in handy.

https://www.nexusmods.com/homefront/mods/3?tab=description

(I haven't tested the modunlocker to see if it could work, but it might)