I would like to have a local Raspbian repo cloned from the official repository. This repo would be hosted on a PC running Windows and would be used by a Raspberry PI that doesn't have an internet connection. Would it be possible to clone the repo from this very same Windows PC and then use the downloaded files to host the repo locally or is it mandatory to first clone the repo from a PC running Linux? If it was possible to do this directly from Windows, how do I do it?
The Raspberry Pi pulls from Debian repos and the Foundation's custom repo which holds RPi specific software. A tutorial on setting up an entire Debian mirror (same instructions should work for the RPi repo) is way outside the scope of this site. I would suggest you look at Debian's documentation on how to setup a repo mirror.
It's worth noting that the entirety of the repo is roughly 1.5 TB. You can cut that down by just including the arm and source stuff which will put you closer to 150 GB.