We are in the process of rewriting  help pages for paypal button but for now see http://help.wildapricot.com/display/DOC/Inserting+HTML+forms and for other specific paypal button codes you would need to use go here https://www.paypal.com/IntegrationCenter/ic_standard_home.html.  I am not sure I quite understand what you are trying to do ...

Yes, you can upload PDF forms using instructions here: http://help.wildapricot.com/display/DOC/Inserting+a+document+into+a+web+page

1. Noted. FYI, right now you can do this with a third party form managers like wufoo.com 2. OK, we will consider this. In your opinion, why is it better / how is it different from simply having a public-accessible page e.g. 'Member benefits'  which would serve as a teaser and provide links to specific premium pages under Members-only section?

We are investigating our options in this direction.

Wow, very impressive! I mean the form itself - and works within WA even though we have not designed specifically for embedding external forms. Though the instructions do not display, you are right :-( I guess some kind of conflict with our own Javascript stuff. We will take a look at that but I do not expect we can do anything about it short ...

We have not tried it ourselves but this might work - try e.g. a wufoo form. Do not see any security issues from the top of my head.