To find out which contacts downloaded a whitepaper, you can add tracking to your landing pages and your external website and then you can query all identified contacts.
Campaign will know a visitor's identity if the visitor clicks through to the landing page through an email that you send out of Campaign or if they filled out a form which associated their web visit data with their contact record.
Without web tracking on your site, any emails that drive contacts to your site will not get identified unless the email pointed them directly to the landing page.
If the visitor is anonymous, the system will record the file name click, but it will not associate the file name until it knows who the contact is.
If your download links are not behind a form submit, it is important that you ensure your lead capture forms are matching the landing page visitor to their contact record. If you are using Campaign forms in landing pages, this will happen automatically. If you have your own forms on your (external) site, you may need to do some additional coding.
To keep things simple and ensure that you know the identity of as many contacts as possible, be sure and put all of your landing pages and forms within a single landing page site in Campaign.