Re: If the lock icon appears in the browser is it safe to assume that a PDF form will send my credit card information securely (over HTTPS)
When you are on a secure page, and a form on the secure page is submitting to an insecure page, Firefox should display a warning and let you cancel the submit. (See attached.)
However, I don't know whether that applies to forms hosted in a plugin, such as Adobe Acrobat. I'm trying to think of a way to test that... (I do not have a version of Adobe that can create a PDF form)
Firefox's built-in PDF preview does not support forms (as of Firefox 19), so that isn't an option either.
However I did notice that if you save the form and open it in the Adobe Reader application (instead of the plugin) it shows a confirmation dialog before sending form data. The confirmation dialog displays the URL so you can look for HTTPS in the URL (but Adobe does not check the integrity of HTTPS certificate).
As best I can tell, PDF forms are not a secure method of submitting credit card information since there is no simple way for the user to independently verify that the form contents will be encrypted before sending.