You cannot combine files once security is set (except using portfolios). You cannot extract pages once security is set. Some security options allow page deletion which is as good as extraction.
You set security from File > Properties using Security. Set PASSWORD SECURITY. You will get a warning about third party software, be sure you fully understand it; you cannot protect from a motivated and informed person who wants to edit your file. And even if you could it is no real protection as they could make a fake form. If this is a real concern look at digital signatures - and no, nobody not even the document''s creator can edit a signed file except in a way that invalidates the signature.