How to add an "apply for a job" button to a content type? I am trying to add an apply button to a content type, like job, so when the user visit the content he could click "apply button", and his resume will be send automatically to the author of the content.

I would prefer this be achieved with AJAX. How can I make something like this?
I would do that using Flag module, create a flag named 'Apply' and set it for job content types. You can then use Flag Actions or Rules to send emails.