Is this just to review? Are they able to comment or ask questions? You could always use DropBox and then just email a link to those customers on plan. That's probably the easiest. A very low percent of customers want to login to anything.
You could even create a Google Doc (or probably some type of Microsoft) and email that and have your DropBox links on the document.
Is it completely secure? Nope. But you're not emailing the nuclear launch keys -- and it's very unlikely one customer would share with another. You'd always be able to change the links if needed, etc.