Is it an NDA (Non-Disclosure Agreement)?
If it is, then you can sign it in an order if it is required for you to complete the order, but you must have an order first. If the buyer won't order before you sign the document then you may have to create a custom gig to cover the NDA though it would be better not to do this.
If it's not an NDA then you will have to be more specific.
You make sure it has no virus the same way you make sure any document that anyone sends you has no virus.
If you're unsure or unhappy then tell the buyer that you're sorry you can't help him with his project.