Can I create a login form that has a document as it's successful reply page?

Sorry for not understanding you clearly.

Do you mean you want to create a login form which will trigger a successful message once it is submitted?

You can create a login form and upon submission you can show a successful message.

You can refer to these links to know more about how you can do this:

https://www.jotform.com/help/2-How-to-Create-Your-First-Web-Form

https://www.jotform.com/help/211-Setting-up-the-Thank-You-Page

If this is not what you are asking of, then please clarify.