When you are running out of space, first try windows disk cleanup first.
If you really need more space, then adding storage must be your first action.
A hard drive addition is relatively inexpensive and easy to install.
It is unlikely that you need more ram unless you regularly have many apps and windows open.
Even if you do, adding 8gb is not guaranteed to work 100% and will not be supported.
Ram must be matched in kits.
Think more of replacing with a 2 x 8gb kit and selling the old ram.
For office work, I would place a second monitor high on my wish list.