Hi Kristen,
To record web page visits, Munchkin is required on the page being visited. Even though we will be able to track downloads of a PDF, we cannot associate this download back to any lead since Munchkin cannot be put on a PDF. PDFs are files and not actual web pages so scripts, such as Munchkin, cannot be loaded on them. Thus, Marketo has no way to track who actually downloaded the PDF.
Instead, we suggest to find leads who downloaded a PDF by using the "Clicked Link in Email" or "Clicked Link on Web Page" filters and selecting the PDF as the link being clicked.
Best,
Will