Kyle,
Good question. You are on the right track to use a marketo form embedded into your WordPress pages.
You can create a Global Whitepaper form, and then what you will do is create a blank landing page with just the form on it. From there you take that URL code and post it into the iFrame on your WordPress site.
You will need to create a unique landing page for each asset, but you can keep one form. From there you just need to set your trigger so it is:
Fills out form: Whitepaper
Web Page is: Blank web page with your Whitepaper form on it.
Attached is a screenshot of what your smart list should look like:
Hope that helps!