Create a partition on the free space and format it with ext4, btrfs, xfs or some other Linux-native filesystem. Then, if you intend to use this free space for personal documents only, set up a mountpoint for it in your home directory, and add a record for this mountpoint to /etc/fstab ─ something along the lines of…
In order to find out the UUID of the partition ─ after you’ve formatted it, of course ─ you first have to look at what partition it is ─ e.g. if the HDD is seen by the system as /dev/sdb and it’s the second partition on that drive, then it would be something like this…
lsblk -o UUID /dev/sdb2
Copy that UUID string and use that in /etc/fstab.
As an aside, I don’t mean to be tooting my own horn, but as it just so happens to be, I’ve written a very extensive tutorial on working with UNIX filesystems and permissions only a few days ago. It’s a long read, but I’ve tried covering all the basics.