I have a web server running currently which is low on free storage space and many more files are incoming. I want to store files from that web server onto another server (NAS, FreeNAS server). An example: someone wants to download a PDF file from the website (on the web server), but the file will be downloaded from the FreeNAS server. Is this possible? Can I use FreeNAS for this?
Yes, it's possible. Deploy FreNAS/NAS. Mount NAS to your web server. Use the mapped storage/volume/directory as storage for your websites. However, this kind of setup is okay for home media/web servers/lab, but for production usage, I would recommend you take more hardware and setup one more web server.