Prepare a Form - not allowing others to sign

Hi, 

I have created a fillable form through the "Prepare a form" tools and followed all instructions on how to use it. Once I have saved this form and send it to someone, via email, to complete it, they cannot open it as it says "Restricted Document - Modifying the document is not allowed because it is a read-only file". I have checked everything in regards to security and protection, but there is none on it, so it should work. 

Following this, I tried to share via "Request e-signatures", but when you go to "Prepare document", it removes any Drop-down list components. 

Can you pleae advise me on what to do?

Regards