As usual Rohit nails it, but there are other ways to do it too, depending on exactly how you have whitepaper downloads set up. For example, you could create contact fields such as "first content downloaded" and "most recent content downloaded" and use form processing steps to write the name or URL of the whitepaper to those fields as appropriate, kind of like "lead source original" and "lead source most recent" - heck, if you consider your whitepapers a lead source, you could even put them in those fields if you wanted. Then of course all you would have to do is edit your integration rules to include those fields in the description that gets passed to Salesforce.
If you're using a single content download form with dynamic content selection of some kind it might be a bit trickier, but there should be a way to use form processing steps and/or update rules to store the data.