Is the .pak file even meant to be opened? Usually when you download firmware files from a manufacturer, they are in a .zip archive file. When you open that .zip archive file you will have the firmware update file and possibly a proprietary utility (usually Windows) to use to upload the firmware file to the device.
Have you read the instructions of the manufacturer on how to upgrade the firmware on those security cameras? Unless they provide a macOS app/utility or an OS independent method of uploading the firmware, then it may not be possible to do so from macOS. I believe some security cameras may allow using a web browser to connect & upload the firmware, others may allow you to upload the firmware file using a simple command line tool available with any OS.
We are working blind here because we have no idea what file(s) you have or how these cameras are meant to be updated.
FYI, just for fun I decided to check out the Reolink website and could not find any firmware downloads, but I did stumble across this Reolink article which includes a video on updating firmware for one of their devices. You do NOT unzip the .pak file, you unzip the ".zip" file which was downloaded. If you used Safari to download the file, then Safari/macOS automatically extracted the contents of the .zip file which was downloaded and left you with just the .zip file (perhaps other browsers have this option to automatically open downloaded files -- terrible option to have enabled IMHO).
https://support.reolink.com/hc/en-us/articles/900004550323-How-to-Upgrade-Firmware-via-Reolink-Client-New-Client-