To answer the question clearly
Am I legally forced to sign this or can I say no as well?
No, you are never forced to sign an NDA.
This should have been done before you began work. This means that if you did not sign the NDA you would typically not be able to begin work. If they ever try to say that you "MUST" sign it you should decline and refuse. BUT always state that if you ever need to do the same or similar work for them again they are within their right to require an NDA.
So with that being said be careful of refusing the NDA as you will likely not be receiving work from that company again unless they absolutely need you again and will then require you to sign another NDA form.
Delving deeper into the subject of NDAs
Most aren't intended to be harmful or malicious to individuals. They just don't want you directly telling another company their trade secrets, which could run them out of business. That's it. There are much worse things to sign such as an NCC, which can very much stop you from going on to another job in the same industry.