If you'd used this pdf in the email as a tracked link (CTA), then you can create a email link performance report with SL set to "Clicked Link in Email" Email is any, and Link contains <pdf url>. In the report you should see the email click link activities of the qualified people (i.e., all the email clicks of the people who'd have clicked the <pdf url>), in the report locate the pdf-url link. Under the pdf-url link you should also see the Email asset name in the Marketo Program Name.Email asset name format (like ABC Test.ABC Test Email in the below snapshot), you should be able to get the Marketo program name with email asset that has the pdf url in it from here.
