The only way I know of installing it is in the documents folder you mentioned, which is on your C drive.
Also, keep in mind many of these mods are available on the workshop, which uses Links instead of downloading to your harddrive. You should try those first.
If there is a mod in the archive which isn't on the workshop, you'll just need to clean out some room on your harddrive. I recommend the tool windirstat, it's very good at identifying where your storage space is getting used the most.